Light emitting module, a lamp, a luminaire and a display device

ABSTRACT

A light emitting module includes at least one LED mounted on a reflective base and surrounded by reflective walls to form a cavity. A partially diffusive layer containing a phosphor is mounted above the LED with a gap therebetween. Some of the light emitted by the phosphor is backscattered toward the LED, base, and walls. The LED absorbs a significant amount of the light, while the reflective base and walls efficiently reflect over 90% of the light back toward the phosphor layer for exiting the module. Various equations are provided that allow the designer to select an optimal gap between the LED and the partially diffusive layer to maximize light extraction efficiency out of the diffusive layer. The optimal gap range for maximizing light extraction efficiency is a product of LED largest linear length, LED surface area, base area, wall area, and reflective coefficients of the various elements.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§371 of International Application No. PCT/IB2012/051405, filed on Mar.23, 2012, which claims the benefit of EP 11181914.0 Sept. 20, 2011.These applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a light emitting module which comprises aluminescent layer and a solid state light emitter. The invention furtherrelates to a lamp, a luminaire and a display device comprising a lightemitting module.

BACKGROUND OF THE INVENTION

Published patent application US2009/0322208A1 discloses a light emittingdevice. A Light Emitting Diode (LED) is provided within a conical cavityformed by a recessed housing. At the front side of the recessed housingthe conical cavity is covered with a transparent thermal conductor layeron which a refractory luminescent layer is provided. At the backplane ofthe recessed housing a heat sink is provided and the side walls of therecessed housing are covered with a metal frame. The conical cavity maybe filled with a material such as silicone.

The LED emits light of a first color towards the luminescent layer. Aportion of the emitted light may be reflected or scattered back into thecavity by the luminescent layer. Another portion of the emitted light isconverted by the luminescent layer into light of a second color. Whenthe luminescent layer emits the light of the second color, this light isemitted in all directions, and thus a part of the light of the anothercolor is emitted into the cavity. Light which is reflected back into thecavity or light of the second color which is emitted into the cavitypartially impinges on a base of the cavity, partially impinges on a wallof the cavity, and partially impinges on the LED. At the surfaces of theLED and at the surfaces of the cavity the light is partially reflectedand partially absorbed. Especially the absorption of light results in aninefficiency of the light emitting device.

Some light module manufacturers provide light emitting modules whichcomprise a cavity with a base. These modules often have a plurality oflight emitters, such as for example LEDs, provided on the base. Incertain embodiments of these light emitting modules the luminescentlayer is provided directly on top of the light emitters, for example viaa bond layer, and in other embodiments the luminescent layer is aso-called remote luminescent layer which means that there is arelatively large distance between the light emitter and the luminescentlayer in the order of centimeters.

A problem of the light emitting modules with the light emitters whichhave the luminescent layer directly on top is that light directed backfrom the luminescent layer to the LED suffers from poor recyclingefficiency due to the fact that back reflectors inside the LED have alimited reflectance (typically the back mirror is silver, with 90%reflectance levels). In reality the actual reflectance is even lower asthe light emitter material, typically GaN/InGaN or AlInGaN, has a highrefractive index, causing light to be trapped inside the light emitterand thus further limiting metal reflectance. Typical LED reflectioncoefficients are close to 70% (averaged over the visible spectral rangeand measured at normal incidence). Another problem of these lightemitting modules is the formation of hot spots in which most of thelight is concentrated in the area on top of the LED and the light outputof the module is therefore highly non-uniform resulting in hot spotsboth in light output and thermal distribution. Furthermore, a phosphorlayer on top of the LED die may get relatively hot and is excited with ahigh flux density, leading to a non-optimal phosphor conversionefficiency thereby limiting the luminescent performance.

The light emitting modules with the remote luminescent layer aregenerally more efficient than the light emitting modules with the lightemitters which have the luminescent layer directly on top, because of amore efficient recycling of light inside the cavity. Also the lightoutput of these modules is typically more homogeneous, reducing the hotspots. However, the light emitting modules with the remote luminescentlayer have a relatively large size compared to the light emittingmodules with the light emitters which have the luminescent layerdirectly on top. The relatively bulky remote luminescent layer solutionscannot be used in size constrained applications, such as spot lampapplications, for example halogen replacement lamps and parabolicreflector lamps.

Another disadvantage of light emitting modules with a remote luminescentlayers is that the relatively large area of the luminescent layerresults in relatively high material cost levels. In addition, the heatconductance within the phosphor layer is only directed laterally towardsthe side walls of the light emitter and due to their bulky construction,the ability to direct the heat away from the remote phosphor plate islimited.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a light emitting modulewhich is relatively efficient.

A first aspect of the invention provides a light emitting module asclaimed in claim 1. A second aspect of the invention provides a lamp asclaimed in claim 23. A third aspect of the invention provides aluminaire as claimed in claim 24. A fourth aspect of the inventionprovides a display device as claimed in claim 25. Advantageousembodiments are defined in the dependent claims.

A light emitting module in accordance with the first aspect of theinvention emits light through a light exit window. The light emittingmodule comprises a base, at least one solid state light emitter and apartially diffusive reflective layer, which is a layer that has diffusereflective properties wherein at least a part of incident light isdiffusively reflected and at least a part of the incident light istransmitted through this layer. The base has a surface and at least partof the surface of the base reflects light that impinges on the surfaceof the base. The part of the surface of the base that reflects light isnamed hereinafter the light reflective surface of the base. The lightreflective surface has a base reflection coefficient which is defined bya ratio between the amount of light that is reflected by the lightreflective surface of the base and the amount of light that impinges onthe light reflective surface of the base. The at least one solid statelight emitter is configured to emit light of a first color range and hasa top surface and a solid state light emitter reflection coefficientwhich is defined by a ratio between the amount of light that isreflected by the at least one solid state emitter and the amount oflight that impinges on the top surface of the at least one solid statelight emitter. A largest linear size of the top surface of the at leastone solid state light emitter is defined as the longest distance from apoint on the top surface of the at least one solid state light emitterto another point on the top surface of the at least one solid statelight emitter along a straight line. The light exit window comprises atleast a part of the partially diffusive reflective layer. A solid statelight emitter area ratio is defined as the ratio between the area of thetop surface of the at least one solid state light emitter and the areaof the light reflective surface of the base. The value of the basereflection coefficient is larger than 70% and larger than the solidstate light emitter reflection coefficient. A gap is present between thetop surface of the at least one solid state light emitter and thepartially diffusive reflective layer. The gap is defined by a distancebetween the top surface of the at least one solid state light emitterand the partially diffusive reflective layer. This distance is in arange with a minimum value equal to or larger than 0.3 times a largestlinear size of the top surface and a maximum value equal to or smallerthan 5 times the largest linear size of the top surface for a relativelysmall value of the solid state emitter area ratio, i.e. smaller than0.1. For intermediate values of the solid state emitter area ratio, i.e.in a range with a minimum value that is larger than or equal to 0.1 anda maximum value that is smaller than or equal to 0.25, the distance isin a range with a minimum value equal to or larger than 0.15 times thelargest linear size of the top surface and a maximum value that is equalto or smaller than 3 times the largest linear size of the top surface.For a relatively large value of the solid state emitter area ratio, i.e.larger than 0.25, the distance is in a range with a minimum value equalto or larger than 0.1 times the largest linear size of the top surfaceand a maximum value that is equal to or smaller than 2 times the largestlinear size of the top surface.

The distance between the top surface of the solid state light emitterand the partially diffusive reflective layer is defined as the length ofthe shortest linear path between the top surface of the at least onesolid state light emitter and the surface of the partially diffusivereflective layer that faces the at least one solid state light emitter.If the light emitting module comprises more than one solid state lightemitters, the distance between the top surfaces of the solid state lightemitters and the partially diffusive reflective layer is an average ofthe distances between each of top surfaces of the solid state lightemitters and the partially diffusive reflective layer.

The largest linear size of the top surface of the solid state lightemitter is defined as the longest distance from a point on the topsurface of the at least one solid state light emitter to another pointon the top surface of the at least one solid state light emitter along astraight line. If the light emitting module comprises more than onesolid state light emitters, the average value of the largest linearsizes of the top surfaces is used. The top surface may be any shape, forexample, a square, rectangle, circle or ellipse. For the square or therectangle, the longest linear distance is the length of a diagonal ofthe square or the rectangle. For the circle, the longest linear size isthe length of a diameter of the circle.

The inventors have found with experiments that the distance between thesolid state light emitter(s) and the partially diffusive reflectivelayer should have a minimum value above which a relatively large lightoutput of the light emitting module is obtained and which depends on thesolid state emitter area ratio. Below this minimum value the lightemitting module operates less efficiently and too much light isreflected, back scattered and/or re-emitted by the partially diffusivereflective layer to the at least one solid state light emitter. Further,the inventors have found that when the distance between the at least onesolid state light emitter and the partially diffusive reflective layerbecomes too large, the light output starts to decrease and is thereforenot advantageous, also depending on the value of the solid state emitterarea ratio. The decrease is the result of more absorption of lightbecause the light has a longer traveling path through the light emittingmodule and thus may experience more absorption events.

The value of the base reflection coefficient is at least larger than thevalue of the solid state light emitter reflection coefficient andconsequently the base absorbs less light than the solid state emitter.This is advantageous because more light is reflected by the base andtherefore more light may be emitted through the light exit window in theambient of the light emitting module. It actually means that more lightis reflected by the base which is subsequently recycled instead ofabsorbed. The efficiency of the light emitting module as a wholeimproves, as light losses in the light emitting module according to theinvention are minimized. Compared to light emitting modules with aluminescent layer directly on top of the solid state light emitter, lesslight is lost by the light absorption of the solid state light emitter.Compared to light emitting modules with a remote luminescent layeracting as a partially diffusive reflective layer, the light that isreflected, back scattered and/or re-emitted by the partially diffusivereflective layer to the inside of the module is recycled moreefficiently as it has less interaction (reflections) inside the modulebefore exiting the light exit window. As a result, a light emittingmodule according to the invention is relatively efficient.

The inventors have found experimentally that the specific combination ofthe base reflection coefficient being higher than the solid state lightemitter reflection coefficient according to the previously specifiedcriterion and the criterion of the distance between the top surface ofthe solid state light emitter(s) and the luminescent layer being in thespecific range, leads to a relatively high light output and thus arelatively efficient light emitting module.

The gap has to be interpreted broadly. The meaning is that the partiallydiffusive reflective layer is not in direct contact with the top surfaceor top surfaces of the at least one solid state light emitter and thatthere is a certain distance between the at least one solid state lightemitter and the partially diffusive reflective layer. The gap may befilled with air, but a substantially transparent material may also bepresent in the gap.

If the partially diffusive reflective layer is not in direct contactwith the top surface of the solid state light emitter or solid satelight emitters, a relatively larger amount of light will be reflectedand emitted towards the light reflective surface. If, according to theinvention, the light reflective surface has a higher reflectivity thanthe at least one solid state light emitter, more light will be reflectedback to the partially diffusive reflective layer and, consequently, ahigher light output will be obtained.

The inventors have found experimentally that the optical effect of arelatively high reflectivity may further increase the light output. Ifthere is a gap between the solid state light emitter(s) and thepartially diffusive reflective layer, the at least one solid state lightemitter does not become as warm as it would be when the partiallydiffusive reflective layer is positioned on top of, or very close to,the solid state light emitter(s). This further improves the efficiencyof the at least one solid state light emitter and may allow a highercurrent loading before a critical temperature is reached in the solidstate light emitter or a solder joint of the solid state light emitter.Hence, a higher absolute light output is realized. Also, if thepartially diffusive reflective layer is not directly thermally coupledto the at least one solid state light emitter it does not receive theheat from the at least one solid state light emitter. It depends on thequality of thermal interface towards the base and a possible heat sinkto which the module is connected how well the partially diffusivereflective layer can be cooled.

Thus, a specific combination of the base reflection coefficient beinghigher than the solid state light emitter reflection coefficientaccording to the presence of the gap, leads to a higher light outputthan one expects only on basis of the optical effect of more reflectionby the light reflective surface of the base.

In an embodiment the partially diffusive reflective layer comprisesluminescent material for converting at least a part of the light of thefirst color range into light of a second color range. The light of thesecond color range is emitted by the luminescent material in alldirections and a part of this light is also emitted towards the at leastone solid state light emitter or towards the light reflective surface ofthe base.

The light conversion from the first spectral range towards the secondspectral range, in case the partially diffusive reflective layer acts asa luminescent layer, converts light energy partially to heat, typicallydenoted as ‘Stokes shift’ losses. Furthermore, in practice the QuantumEfficiency (QE) of the luminescent material(s) is limited, e.g. to 0.9giving rise to further thermal heat-up of the partially diffusivereflective layer comprising luminescent material, hereafter called aluminescent layer. The efficiency of the luminescent material is higherif the temperature of the luminescent material is kept within acceptablelimits. This can be achieved by limiting the light flux loading, i.e.the flux density distribution, on the luminescent material, for exampleby applying a specific distance between the solid state light emittersand the luminescent layer thus allowing the light to spread therebyreducing the flux density on the luminescent layer. However, morepreferably the thermal resistance between the luminescent layer and thebase and between the luminescent layer and the heat sink is optimized toachieve a low thermal resistance. This can be achieved by various means,such as by coupling the luminescent layer to a heat conductive wall atthe circumference of the exit window, or by applying a heat conductivematerial between the emitters and the base and the luminescent material,such as a heat conductive glass or ceramic or by applying heat spreadinglayers or structures on the luminescent layer, such as a carriersubstrate to which the luminescent layer is attached with heatconductive properties. Thus, with such measures, the gap between thesolid state light emitter(s) and the luminescent layer results in thephoto-thermal effect of a more efficient luminescent layer.

Further, the gap between the at least one solid state light emitter andthe partially diffusive reflective layer results in a more uniformdistribution of light flux through the partially diffusive reflectivelayer instead of a relatively high light flux in a very specific area ofthe partially diffusive reflective layer. Also a reduction of thermalhot spots and temperature gradients is achieved in this way in case thepartially diffusive reflective layer comprises a luminescent material.Luminescent materials tend to be sensitive to photosaturation, whichmeans that above a certain light flux, the luminescent material convertslight at a lower efficiency. Thus, by having a gap between the solidstate light emitter(s) and the partially diffusive reflective layercomprising luminescent material, photosaturation of the luminescentmaterial is prevented and efficiency is improved.

In an embodiment the top surface of the at least one solid state lightemitter faces towards the light exit window. In an embodiment one of thesolid state light emitters is a socalled side emitter. In an embodiment,at least one solid state light emitter emits light towards at least apart of the light exit window.

In an embodiment, the light emitting module comprises a plurality ofsolid state light emitters. Each one of the solid state light emittersis configured to emit light in a specific color range. In anotherembodiment the plurality of solid state light emitters are provided onan imaginary plane which is in between the base and the light exitwindow. In a further embodiment at least one of the plurality of solidstate light emitters emits light towards at least a specific part of thelight exit window. Additionally or alternatively, at least one of theplurality of solid state light emitters has a top surface facing towardsthe light exit window. The solid state light emitter reflectioncoefficient is defined as the average value of the reflectioncoefficients of the plurality of solid state light emitters. In afurther embodiment the top surface of at least one solid state lightemitter faces the light exit window and the top surface of another solidstate light emitter does not face the light exit window.

In specific embodiments, the light emitter may be a combination of aplurality of solid state light emitters with their light emittingsurfaces positioned very close to each other in one plane. Very closemeans that the distance between the individual solid state lightemitters is in the order of tens of micrometers, but not more than 0.2mm. Such closely positioned solid state light emitters are seen in thecontext of this invention as a single light emitter, also called amulti-die LED. The top surface is the combination of top surfaces of theindividual solid state light emitters of the very closely positionedsolid state light emitters. It is to be noted that the very closeplacement relates to the dies of the solid state light emitters and notto the very close placement of packages of solid state light emitter.

The light emitting module is able to emit more light if more than onesolid light emitter is provided. More light, seen in absolute values,will be reflected within the light emitting module and consequentlyemitted back towards the solid state light emitters and the lightreflective surface of the base. Thus, if the light reflective surface ofthe base has a better reflectivity than the solid state light emitters,more light, seen in absolute values, may be recycled by reflecting thelight via the reflective surface back to the partially diffusivereflective layer (and through the light exit window). Further, the lightemitting module with a plurality of solid state light emitters has thesame advantages as the light emitting module with a single solid statelight emitter. In the case of two or more solid state light emitters thetotal summed area of the top surfaces of the solid state light emittersis used in the calculation of the solid state light emitter area ratio.

In an embodiment the value of the base reflection coefficient is largerthan the solid state light emitter reflection coefficient plus a factorc times the difference between 1 and the solid state light emitterreflection coefficient. The value of the factor c depends on the valueof a solid state light emitter area ratio which is defined as the ratiobetween the area of the top surface of the at least one solid statelight emitter and the area of the light reflective surface of the base.If the solid state light emitter area ratio is relatively small, i.e. avalue that is smaller than 0.1, then a relatively efficient lightemitting module is provided if the factor c is larger than or equal to0.2. If the solid state light emitter area ratio is in an intermediaterange, i.e. in a range with a minimum value that is larger than or equalto 0.1 and a maximum value that is smaller than or equal to 0.25, then arelatively efficient light emitting module is provided if the factor cis equal to or larger than 0.3. If the solid state light emitter arearatio is relatively large, i.e. has a value that is larger than 0.25,then a relatively efficient light emitting module is provided if thefactor c is equal to or larger than 0.4. The factor c has a maximumvalue of 1, because a reflection coefficient value cannot be largerthan 1. In practice the value of the solid state emitter area ratioranges between 0 and 1.

Light of the first color range which impinges on the partially diffusivereflective layer is scattered and partially reflected towards the atleast one solid state light emitter and the base due to reflections by asurface of the partially diffusive reflective layer and due to internalreflections and due to back scattering in the partially diffusivereflective layer, and is also partially transmitted through thepartially diffusive reflective layer.

The at least one solid state light emitter has a limited solid statelight emitter reflection coefficient due to its construction, whichmeans that a significant portion of the light which impinges on the atleast one solid state light emitter is absorbed by the at least onesolid state light emitter. The top surface of the at least one solidstate light emitter reflects a relatively small portion of the lightwhich impinges on the top surface, and a relatively large portion ofthat light is transmitted into the core of the solid state lightemitter. The back surface and the semiconductor regions inside the solidstate light emitter absorb a significant portion of the light and, as aconsequence, a limited amount of light, which enters into the core ofthe solid state light emitter, is emitted back into the ambient of thesolid state light emitter. Often the word ‘die’ is used for solid statelight emitter chip and both terms refer to the semiconductor device inwhich the light is generated. The semiconductor device includes thesemiconductor material which actually generates the light, and alsoincludes electrode, segmentation, vias, back side mirrors, and forexample, protection layers. It is to be noted that in some applicationssolid state light emitters are grown on a light transmitting substrate,for example, sapphire. After the manufacturing, the substrate may stillbe present on the solid state light emitter die and the light which isgenerated in the solid state light emitter is emitted through the growthsubstrate. The term ‘top surface’ does not refer to a surface of thegrowth substrate, but to a surface of the solid state light emitter diewhich emits most of the light. In some embodiments the light emissionthrough the top surface is mainly in the direction of the light exitwindow.

It was noticed that, if the base reflection coefficient is sufficientlyhigher than the solid state light emitter reflection coefficient, theefficiency of the light emitting module as a whole substantiallyimproves. Further, a significant improvement was noticed above a certaindifference in reflection coefficients dependant on the solid state lightemitter area ratio. Thus, according to this embodiment, the basereflection coefficient is at least larger than the value of the solidstate light emitter reflection coefficient plus a factor c times thedifference between 1 and the value of the solid state light emitterreflection coefficient. If it is assumed that Rbase is the basereflection coefficient and R_SSL is the solid state light emitterreflection coefficient, then this criterion is represented by theformula: Rbase>R_SSL+c*(1−R_SSL). Thus, if the solid state emitter arearatio is relatively small, i.e. smaller than 0.1, meaning that thereflective surface of the base has a relative large area with respect tothe area of the top surface of the solid state light emitter, then arelatively efficient lighting module is provided for if c≧0.2. As anexample, if in this case R_SSL=0.7, then the reflection coefficient ofthe reflective surface of the base should be larger than or equal to0.76 to achieve a relatively efficient light emitting module. If thesolid state emitter area ratio is in an intermediate range, i.e. is in arange with a minimum value that is larger than or equal to 0.1 and amaximum value that is smaller than or equal to 0.25, meaning that thearea of the reflective surface of the base is comparable to the area ofthe top surface of the solid state light emitter, then a relativelyefficient lighting module is provided for if c≧0.3. As an example, if inthis case R_SSL=0.7, then the reflection coefficient of the reflectivesurface of the base should be larger than or equal to 0.79 to achieve arelatively efficient light emitting module. If the solid state emitterarea ratio is relatively large, i.e. is larger than 0.25, meaning thatthe reflective surface of the base has a relatively small area withrespect to the area of the top surface of the solid state light emitter,then the factor c should be larger than or equal to 0.4 to achieve arelatively efficient light emitting module. As an example, if in thiscase R_SSL=0.7, then the reflection coefficient of the reflectivesurface of the base should be larger than or equal to 0.82 to providefor a relatively efficient light emitting module.

It should be noted that the reflection coefficients are average numbersover a whole surface to which they relate. The light reflective surfaceof the base may comprise, for example, areas which are less reflectivethan other areas, such as by using different materials and/or differentreflector layer thicknesses on the base. Further, the reflection oflight of different wavelengths may differ, however, preferably thereflection coefficient is a weighted average over a spectral range,which comprises at least light of the first color range, and over anangle of incidence distribution.

In some cases the at least one solid state light emitter is attached toa substrate, for example, a ceramic substrate, and the combination ofthe substrate and the at least one solid state light emitter is attachedto another carrier layer. This carrier layer may for instance be a metalcore printed circuit board (MCPCB) also called insulated metal substrate(IMS) or a conventional PCB, such as FR4, or another ceramic carrier,such as alumina or aluminiumnitride. In such situations, the base of thelight emitting module is the combination of the another carrier layerand the substrate to which the at least one solid state light emitter isattached. In other words, the base is the combination of materialsand/or layers on which solid state light emitter(s) are provided.Consequently, in this specific case, the base reflection coefficient isthe weighted average of reflection coefficients of the substrates andthe carrier layer. For the avoidance of doubt, in the calculations thearea of the reflective surface of the base does not include the areathat is covered by the at least one solid state light emitter.

When the solid state emitter area ratio is relatively small, i.e.smaller than 0.1, then a more efficient light emitting module isobtained in case 0.4≦c≦1. An even more efficient light emitting moduleis obtained in this case for 0.6≦c≦1. When the solid state emitter arearatio is in an intermediate range, i.e. is in a range with a minimumvalue that is larger than or equal to 0.1 and a maximum value that issmaller than or equal to 0.25, then a more efficient light emittingmodule is obtained in case 0.6≦c≦1. An even more efficient lightemitting module is obtained in this case for 0.84≦c≦1. If the solidstate light emitter area ratio is relatively large, i.e. larger than0.25, then a more efficient light emitting module is obtained in case0.8≦c≦1.

In an embodiment the at least one solid state light emitter is providedon the light reflective surface of the base. For the avoidance of doubt,in the calculations the area of the reflective surface of the base doesnot include the area that is covered by the at least one solid statelight emitter. However, in other embodiments the at least one solidstate light emitter may be positioned on a network of wires which areprovided in between the base and the light exit window. In such anembodiment, the wires carry the solid sate light emitter(s) and providepower to the solid sate light emitter(s). The wire may contain a metalcore and a protective plastic cladding and only be electrically attachedat point of contact to the substrate or carrier of the emitter, e.g. bya solder joint connection.

In an embodiment, the light emitting module comprises a wall interposedbetween the base and the light exit window. The base, the wall and thelight exit window enclose a cavity. The wall comprises a lightreflective wall surface facing towards the cavity and the lightreflective wall surface has a wall reflection coefficient which isdefined by a ratio between the amount of light that is reflected by thelight reflective wall surface and the amount of light that impinges onthe light reflective wall surface. In this embodiment an effectivereflection coefficient is defined as a weighted average of the base andthe wall reflection coefficient, for example weighted corresponding tothe sizes of the respective surface areas. In this embodiment theeffective reflection coefficient is at least larger than 70% and largerthan the solid state light emitter reflection coefficient. Thus, thelight emitting module is relatively efficient if the base and the wallscombined have the effective reflection coefficient as specified.

In a further embodiment the effective reflection coefficient is at leastlarger than the solid state light emitter reflection coefficient plusthe factor c times the difference between 1 and the solid state lightemitter reflection coefficient. The criteria for the factor c aresimilar as for the embodiment without the walls, the only differencebeing that the total reflective surface now comprises the reflectivesurface of the wall and the reflective surface of the base. Thus, thesolid state emitter coverage ratio is now defined as the ratio betweenthe area of the top surface of the at least one solid state lightemitter and the sum of the area of reflective surface of the base andthe area of the reflective wall surface. It is to be noted that, in linewith the base and solid state light emitter reflection coefficient, thewall reflection coefficient is a weighted average of reflection of lightof a predefined spectrum of light. It is to be noted that the walls mayhave a further function, such as conducting heat from the partiallydiffusive reflective layer, which comprises, in this example,luminescent material, towards the base. The base is often coupled to aheat sink and the luminescent layer may become relatively hot as theresult of heat generation in case light of the first color range isconverted to light of the second color range. The reflective surface ofthe walls aids in achieving a relatively efficient light emittingmodule.

In an embodiment, the wall reflection coefficient, i.e. the reflectioncoefficient of the walls, is at least smaller than 95% and the distancebetween the top surface of the solid state light emitter and thepartially diffusive reflective layer is in a range with a minimum valueof 0.3 times the largest linear size of the top surface (or the averagevalue of the largest linear sizes of the top surfaces) and a maximumvalue smaller than 0.75 times the largest linear size of the top surface(or the average value of the largest linear sizes of the top surfaces)for a relatively small value of the solid state emitter area ratio, i.e.smaller than 0.1. For intermediate values of the solid state emitterarea ratio, i.e. in range with a minimum value that is larger than orequal to 0.1 and a maximum value that is smaller than or equal to 0.25,the distance in this case is in a range with a minimum value of 0.15times the largest linear size of the top surface (or the average valueof the largest linear sizes of the top surfaces) and a maximum valuesmaller than 0.3 times the largest linear size of the top surface (orthe average value of the largest linear sizes of the top surfaces). Fora relatively large value of the solid state emitter area ratio, i.e.larger than 0.25, the distance in this case is in a range with a minimumvalue of 0.1 times the largest linear size of the top surface (or theaverage value of the largest linear sizes of the top surfaces) and amaximum value smaller than 0.2 times the largest linear size of the topsurface (or the average value of the largest linear sizes of the topsurfaces). The inventors have found that for these criteria a relativelyefficient light emitting module is obtained.

In an embodiment, the wall reflection coefficient is larger than orequal to 95% and a relatively efficient light emitting module isobtained if the distance between the top surface of the solid statelight emitter and the partially diffusive reflective layer is in a rangewith a minimum value of 0.75 times the largest linear size of the topsurface (or the average value of the largest linear sizes of the topsurfaces) and a maximum value of 2 times the largest linear size of thetop surface (or the average value of the largest linear sizes of the topsurfaces) for a relatively small value of the solid state emitter arearatio, i.e. smaller than 0.1. For intermediate values of the solid stateemitter area ratio, i.e. in a range with a minimum value that is largerthan or equal to 0.1 and a maximum value that is smaller than or equalto 0.25, the distance in this case is in a range with a minimum value of0.3 times the largest linear size of the top surface (or the averagevalue of the largest linear sizes of the top surfaces) and a maximumvalue of 0.7 times the largest linear size of the top surface (or theaverage value of the largest linear sizes of the top surfaces). For arelatively large value of the solid state emitter area ratio, i.e.larger than 0.25, the distance in this case is in a range with a minimumvalue of 0.2 times the largest linear size of the top surface (or theaverage value of the largest linear sizes of the top surfaces) and amaximum value of 0.5 times the largest linear size of the top surface(or the average value of the largest linear sizes of the top surfaces).

In an embodiment of the invention at least a part of the reflective basesurface is closer to the partially diffusive reflective layer than thetop surface of the solid state light emitter. In this embodiment anefficient light emitting module is obtained if the distance between thetop surface and the partially diffusive reflective layer is in a rangewith a minimum value of 0.4*d_(SSL)+Δh/2 and a maximum value of5*d_(SSL)+Δh/2 for a solid state light emitter area ratio smaller than0.1, a minimum value of 0.15*d_(SSL)+Δh/2 and a maximum value of3*d_(SSL)+Δh/2 for a solid state light emitter area ratio that is in arange with a minimum value that is larger than or equal to 0.1 and amaximum value that is smaller than or equal to 0.25, or a minimum valueof 0.1*d_(SSL)+Δh/2 and a maximum value of 2* d_(SSL)+Δh/2 for a solidstate light emitter area ratio larger than 0.25. The parameter d_(SSL)is the largest linear size of the top surface of the at least one solidstate light emitter and the parameter Δh is the absolute value of thedifference between the distance between the top surface of the at leastone solid state light emitter and the partially diffusive reflectivelayer and the distance, or the average distance, between the reflectivebase surface and the partially diffusive reflective layer. In case of aplurality of solid state light emitters, average values are used. Inthis embodiment the base has, for example, one or more recesses whereinthe solid state light emitter(s) are placed.

In an embodiment, the wall comprises at least one of the followingmaterials: aluminium, copper, ceramic like alumina, thermally conductivepolymers such as polyamides or spectralon.

In another embodiment, at least one of the light reflective surface ofthe base and/or the light reflective wall surface comprise a lightreflective coating, a light reflective molding, a light reflectiveceramic or a light reflective foil. A light reflective coating may beused to increase the reflectivity of the respective light reflectivesurfaces, thereby improving the efficiency of the light emitting module.In a preferred embodiment, the light reflective surface of the baseand/or the wall diffusely scatter light, which may be obtained by meansof a white coating. A diffusely scattering surface further improves thelight recycling efficiency of the light emitting module. In anotherembodiment, the light reflective surface of the base and/or the wall maybe specularly reflecting, which may be obtained by means of a metalmirror (e.g. protected silver or aluminium). In a further embodiment,the light reflective surface of the base and/or the wall may be acombination of a diffusely scattering material and a specularlyreflecting material.

In a further embodiment, the light reflective wall surface is tiltedwith respect to a normal axis of the base for increasing the reflectionof light towards the light exit window. In another further embodiment,the light reflective wall surface is curved for increasing thereflection of light towards the light exit window. Such a tilted wallsurface or curved wall surface results in a convex cavity, seen from theinterior of the cavity. Further, the tilting or the curving is such thatthe edges of the light reflective wall surface that touch the base arecloser to each other than the edges of the light reflective wall surfacethat touch the partially diffusive reflective layer. The convex cavitywith such a tilted or curved light reflective wall surface betterreflects the light which impinges on the light reflective wall surfacetowards the partially diffusive reflective layer (and thus the lightexit window). It is at least partly prevented that light is reflected bythe light reflective wall surface to the interior of the cavity whichresults in more absorption at another reflection point or by the solidsate light emitter. Consequently, the efficiency of the light emittingmodule increases. This is especially advantageous at a relatively highvalue of the solid state light emitter area ratio.

In an embodiment, the partially diffusive reflective layer forms thelight exit window. The partially diffusive reflective layer has an edge,and the edge of the partially diffusive reflective layer is in contactwith the base. A construction according to the embodiment prevents theuse of walls between the partially diffusive reflective layer and thebase, which may be advantageous in certain applications. In thisembodiment the cavity is formed by the light exit window and the base.Further, it may result in a wider angular light output distribution.

In another embodiment, the light emitting module comprises asubstantially transparent material arranged between the one or moresolid state light emitter(s) and the luminescent layer, the transparentmaterial being optically coupled to the one or more solid state lightemitter(s). The substantially transparent material assists in theoutcoupling of light from the solid state light emitter. The material ofa solid state light emitter has in general a relatively high refractiveindex, and as such a significant amount of light is caught within thesolid state light emitter because of total internal reflection (TIR).The substantially transparent material has a refractive index that iscloser to the refractive index of the solid state light emitter than therefractive index of, for example, air, and as a consequence more lightis emitted into the transparent material and, consequently, finally outof the light emitting module. The transparent material may have arefractive index close to the refractive index of the solid state lightemitter. If the solid state light emitter is of the type of InGaNmaterials, the refractive index of the emitter is close to 2.4 and ahigh refractive index glass or ceramic attached to the emitter surfacewill extract most light from the chip. The transparent material maycomprise various materials applied in various layers or as mixtures. Forexample, a high refractive index ceramic substrate may be bonded with ahigh index glass or a high index resin to the at least one solid statelight emitter. The substantially transparent material may be, forexample, a dome or a flat encapsulant placed on the at least one solidstate light emitter. In an embodiment, the refractive index of thetransparent material is higher than 1.4. In another embodiment, therefractive index of the transparent material is higher than 1.7.

In a further embodiment, the substantially transparent material isoptically and thermally coupled to the luminescent layer. For example,the whole space between the base and the partially diffusive reflectivelayer is filled with the transparent material, and thus, the transparentmaterial is also optically coupled to the partially diffusive reflectivelayer resulting in less reflection at the interface between thepartially diffusive reflective layer and the cavity. Consequently, morelight is emitted into the environment of the light emitting module.Further, if the transparent material is in contact with the partiallydiffusive reflective layer, the transparent material is also thermallycoupled to the partially diffusive reflective layer and assists in theheat conduction from the partially diffusive reflective layer towards,for example, the base. It results in a less warm partially diffusivereflective layer, which is, in general, more efficient and has a longerlifetime. For example, in case the partially diffusive reflective layeris a luminescent layer the transparent material thus provides anenhanced thermal contact between the luminescent material and the basecompared to an air gap. As air has a thermal conductivity of about 0.025W/mK, a silicone resin with thermal conductivity of about 0.3 W/mK willprovide a better thermal interface, whereas a glass substrate likesodalime glass of about 1.0 W/mK thermal conductivity is even better,whereas a borosilicate glass or a fused silica glass of about 1.3 W/mK,a translucent polycrystalline alumina substrate of about 30 W/mK, and asapphire substrate of 42 W/mK are much better. Optionally thesubstantially transparent material may be sintered translucentpolycrystalline alumina wherein the grain size is preferably larger than44 um or preferably smaller than 1 um to provide for a relatively hightranslucency combined with a very good thermal performance.

In another embodiment, the substantially transparent material comprisesat least one of: a transparent resin, a transparent gel, a transparentliquid, a transparent glass, a transparent polymer, and a transparentceramic. Transparent refers to the absence of substantial lightabsorption in the spectral region of the first and second wavelengthrange. Some limited levels of scattering may be allowed in thetransparent layers, especially if this scattering is of a forwardscattering type. Hence, some scattering centers may be allowed in thesubstantially transparent material in between the luminescent materialand the base, for example by using a translucent layer of a slightlyhazy material.

In a further embodiment, the luminescent material comprises at least oneof: an inorganic phosphor, an organic phosphor, a ceramic phosphor andquantum dots, or another fluorescent material, or a mixture of these. Itis to be noted that the luminescent layer may comprise a carrier layer,for example a glass substrate, and a layer of luminescent material, orthat the luminescent layer comprises randomly distributed particles ofthe luminescent material in a carrier layer, or in the case of a ceramicphosphor, substantially the whole luminescent layer is the luminescentmaterial. It is also noted that the luminescent layer may consist ofvarious separate luminescent layers stacked or closely spaced. Differentluminescent materials may be used in the different layers. However, theluminescent materials may also be mixed together in the same layers(s).Examples of inorganic luminescent materials may include, but are notlimited to, Ce doped YAG (Y₃Al₅O₁₂) or LuAG (Lu₃Al₅O₁₂). Ce doped YAGemits yellow light, and Ce doped LuAG emits yellow-green light. Examplesof other inorganic luminescent materials which emit red light mayinclude, but are not limited to ECAS (ECAS, which isCa_(1-x)AlSiN₃:Eu_(x); with 0≦x≦1; especially x≦0.2) and BSSN (BSSNE,which is Ba_(2-x-z)M_(x)Si_(5-y)AlyN_(8-y)O_(y):Eu_(z) (M=Sr, Ca; 0≦x≦1,especially x≦0.2; 0≦y≦4, 0.0005≦z≦0.05).

In an embodiment, the light exit window further comprises a diffuserlayer for obtaining a diffuse light emission, for obtaining a spatially,color and color over-angle uniform light emission, and for obtaining acolor mixed light emission. The light exit window may also comprise adichroic layer for correcting color over angle variations or lightuniformity. In addition to influencing the light emissioncharacteristics by the luminescent layer, other optical layers may alsobe used to influence the characteristics of the light that is emittedthrough the light exit window into the environment of the light emittingmodule, such as for example an optical element for providing a desiredlight beam shape.

In an embodiment a diffuser layer for obtaining a diffuse lightemission, for obtaining a spatially, color and color over-angle uniformlight emission, and for obtaining a color mixed light emission isprovided at a distance from a side of the partially diffusive reflectivelayer facing away from the at least one solid state light emitter.

In an embodiment a polarizing element is positioned at a side of thepartially diffusive reflective layer facing away from the at least onesolid state light emitter.

According to a second aspect of the invention a lamp is provided whichcomprises the light emitting module according to the invention. The lampmay comprise a plurality of light emitting modules. The lamp maycomprises a retrofit light bulb, a retrofit parabolic aluminizedreflector (PAR) lamp, a spot lamp, a downlighter lamp, a retrofithalogen lamp or a retrofit light tube.

According to a third aspect of the invention, a luminaire is providedwhich comprises a light emitting module according to the invention orwhich comprises a lamp according to the invention. The luminaire maycomprise a plurality of light emitting modules.

According to a fourth aspect of the invention a display device isprovided which comprises the light emitting module according to theinvention. In use, the light emitting module may act as a backlightingunit for a LCD display device. As the light emitting module generatesrelatively efficient (polarized light), the cost level of the displaydevice is reduced.

The lamp, luminaire and the display device according to, respectively,the second, third and fourth aspect of the invention provide the samebenefits as the light emitting module according to the first aspect ofthe invention and have similar embodiments with similar effects as thecorresponding embodiments of the light emitting module.

In this context, light of a color range typically comprises light havinga predefined spectrum. The predefined spectrum may, for example,comprise a primary color having a specific bandwidth around a predefinedwavelength, or may, for example, comprise a plurality of primary colors.The predefined wavelength is a mean wavelength of a radiant powerspectral distribution. In this context, light of a predefined color alsoincludes non-visible light, such as ultraviolet light. The light of aprimary color, for example, includes Red, Green, Blue, Yellow and Amberlight. Light of the predefined color may also comprise mixtures ofprimary colors, such as Blue and Amber, or Blue, Yellow and Red. It isto be noted that the first color range may also comprise light which isinvisible for the human eye, such are ultra violet light or infraredlight. The terms “violet light” or “violet emission” especially relatesto light having a wavelength in the range of about 380-440 nm. The terms“blue light” or “blue emission” especially relates to light having awavelength in the range of about 440-490 nm (including some violet andcyan hues). The terms “green light” or “green emission” especiallyrelate to light having a wavelength in the range of about 490-560 nm.The terms “yellow light” or “yellow emission” especially relate to lighthaving a wavelength in the range of about 560-590 nm. The terms “orangelight” or “orange emission” especially relate to light having awavelength in the range of about 590-620 nm. The terms “red light” or“red emission” especially relate to light having a wavelength in therange of about 620-750 nm. The terms “amber light” or “amber emission”especially relate to light having a wavelength in the range of about575-605 nm. The terms “visible” light or “visible emission” refer tolight having a wavelength in the range of about 380-750 nm.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

It will be appreciated by those skilled in the art that two or more ofthe above-mentioned embodiments, implementations, and/or aspects of theinvention may be combined in any way deemed useful.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Modifications and variations of the light emitting module, lamp,luminaire, and/or display device which correspond to the describedmodifications and variations of the light emitting module, can becarried out by a person skilled in the art on the basis of the presentdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIGS. 1a and 1b schematically show cross-sections of embodiments of alight emitting module according to the invention,

FIGS. 2a and 2b schematically show a top-view of embodiments of a lightemitting module according to the invention,

FIG. 3a schematically shows an embodiment of a light emitting moduleaccording to the invention comprising a cavity,

FIG. 3b schematically shows an embodiment of a light emitting moduleaccording to the invention having a cylindrical shape,

FIGS. 4a and 4b schematically show embodiments of a cross-section of theembodiment of a light emitting module according to the inventioncomprising the cavity,

FIGS. 5a and 5b schematically show a plurality of cross-sections ofembodiments of the light emitting module according the invention,

FIG. 6 schematically shows a plurality of cross-sections of embodimentsof light emitting modules according to the invention with a luminescentlayer forming the light exit window and the edge of the luminescentlayer touching the base,

FIGS. 7a and 7b schematically show cross-sections of embodimentsaccording to the invention of a flexible light emitting module,

FIGS. 8a, 8b and 8c schematically show cross-sections of embodiments ofa light emitting module according to the invention,

FIG. 9 shows a graph with the results of measurements of an embodimentof a light emitting module according to the invention,

FIG. 10a shows a schematic cross-section of a first reference lightemitting module,

FIG. 10b shows a schematic cross-section of a second reference lightemitting module,

FIG. 10c shows a schematic cross-section of a light emitting moduleaccording to the invention,

FIGS. 11, 12, 13, 14 and 15 show graphs with the results of simulationsof an embodiment of the light emitting module according to theinvention,

FIGS. 16a and 16b schematically show cross-sections of embodiments of alight emitting module according to the invention,

FIG. 16c shows a graph with the results of simulations of theembodiments of the light emitting module according to FIGS. 16a and 16b,

FIG. 17a schematically shows a top view of an embodiment of a lightemitting module according to the invention,

FIGS. 17b, 17c and 17d show three other graphs with the results ofsimulations of the embodiment of the light emitting module according toFIG. 17 a,

FIG. 18 shows another graph with the results of simulations of the lightemitting module according to the invention,

FIGS. 19a and 19b show two embodiments of lamps according to an aspectof the invention,

FIG. 19c shows an embodiment of a luminaire according to an aspect ofthe invention,

FIG. 20 schematically shows an embodiment of a cross-section of theembodiment of the light emitting module according to the invention, and

FIG. 21 shows an embodiment of a display device according to an aspectof the invention.

It should be noted that items denoted by the same reference numerals indifferent Figures have the same structural features and the samefunctions, or are the same signals. Where the function and/or structureof such an item have been explained, there is no necessity for repeatedexplanation thereof in the detailed description.

The figures are purely diagrammatic and not drawn to scale. Particularlyfor clarity, some dimensions are exaggerated strongly

DETAILED DESCRIPTION OF THE EMBODIMENTS

A first embodiment is shown in FIG. 1a which shows a cross-section of alight emitting module 100 according to the first aspect of theinvention. The light emitting module 100 has a light exit window 104.The light exit window 104 in this embodiment is formed by a luminescentlayer 102 which comprises luminescent material. The luminescent materialconverts at least a part of light of a first color range 114 whichimpinges on the luminescent material into light of a second color range116. At another side of the light emitting module 100 a base 110 isprovided which has a light reflecting surface 112 that faces towards thelight exit window 104. On the base 110 a solid state light emitter 108is provided which emits, in use, light of the first color range 114towards a part of the light exit window 104. The base is typicallyprovided with electrode structures to contact the dies or plurality ofdies of the solid state light emitter 108 to provide electrical power.The electrode structures are not shown in the Figures. The surface ofthe base 110 which is covered by the solid state light emitter 108 isnot included in the light reflective surface 112 of the base 110.

The light reflective surface 112 has a base reflection coefficientRbase, which is defined by a ratio between an amount of light that isreflected by the light reflective surface 112 and an amount of lightthat impinges on the light reflective surface 112. The solid state lightemitter 108 has a solid state light emitter reflection coefficientR_SSL, which is defined by a ratio between an amount of light that isreflected by the solid state light emitter 108 and an amount of lightthat impinges on solid state light emitter 108. It is noted that thereflection coefficients both are an average of the reflectioncoefficients related to light of different wavelengths, for example, an(weighted) average over light of the first color range 114 and light ofthe second color range 116.

The luminescent layer 102 is not positioned directly on a top surface106 of the solid state light emitter 108, but it is arranged at adistance h from the solid state light emitter 108. If the solid statelight emitter 108 emits light of the first color range 114, at least apart of the light of the first color range 114 is reflected by theluminescent layer 102 towards the base 110 and the solid state lightemitter 108. The part of the light of the first color range 114 isreflected by the luminescent layer 102 because of reflection at asurface at which the light impinges, or because of internal reflectionor backscattering. The light which is reflected back, partially impingeson the solid state light emitter 108 and partially impinges on the lightreflective surface 112 of the base 110.

Another part of the light of the first color range 114 may betransmitted through the luminescent layer 102 into the environment ofthe light emitting module 100. A further part of the light of the firstcolor range 114 is converted by the luminescent material into light ofthe second color range 116. The luminescent material emits the light ofthe second color range 116 in a plurality of directions and,consequently, a part of the light of the second color range 116 isemitted into the environment of the light emitting module 100, andanother part of the light of the second color range 116 is emittedtowards the base 110 and the solid state light emitter 108.

The light which impinges on a top surface 106 of the solid state lightemitter 108 is partially reflected and partially transmitted into thesemiconductor material of the solid state light emitter 108. Inside thesolid state light emitter 108 a part of the light is absorbed and someother part of the light is reflected back towards the top surface 106and emitted back towards the light exit window 104. The value of thesolid state light emitter reflection coefficient R_SSL defines whichpart of the impinging light is reflected back, and the value (1−R_SSL)defines how much of the impinging light is absorbed by the solid statelight emitter 108. In practice, the solid state light emitter 108 has arelatively low value of the solid state light emitter reflectioncoefficient R_SSL, generally in the order of 0.7.

The light which is reflected, scattered, i.e. diffusive reflected, oremitted by the luminescent layer towards the base 110 and which does notimpinge on the solid state light emitter 108 is to a large extentreflected by the light reflective surface 112 of the base 110. However,a small amount of light may still be absorbed at the surface or in theunderlying layers. The base reflection coefficient Rbase defines whichpart of the impinging light is reflected back by the light reflectivesurface 112, and the value (1−Rbase) defines how much of the impinginglight is absorbed by the light reflective surface 112.

The value of the base reflection coefficient Rbase and the solid statelight emitter reflection coefficient R_SSL is always a value between 0and 1. It is to be noted that the amount of light which is generated bythe solid state light emitter 108 is not taken into account whendetermining the solid state light emitter reflection coefficient R_SSL.The part of light which is reflected is a part of the amount of lightwhich impinges on the solid state light emitter 108.

According to the invention, the value of the base reflection coefficientRbase is at least larger than the value of the solid state light emitterreflection coefficient R_SSL. Preferably, the value of the basereflection coefficient Rbase is at least larger than the value of thesolid state light emitter reflection coefficient R_SSL plus a factor ctimes the difference between 1 and the solid state light emitterreflection coefficient R_SSL. Thus, Rbase>R_SSL+c·(1−R_SSL). Thus, thelight reflective surface 112 is, on average, more light reflective thanthe solid state light emitter 108 with a value that is at least a valueof c times the difference between a full reflective solid state lightemitter, i.e. a reflectivity of 100%, and the actual reflectivity of theused solid state light emitter 108. The factor c is dependent on thetotal area of the solid state light emitter 108 with respect to thetotal reflective area of the base 110, which is hereafter called thesolid state light emitter area ratio ρ_(SSL):ρ_(SSL)=(A_SSL/Abase), inwhich A_SSL represents the total area of the top surface 106 of thesolid state light emitter 108 and Abase represents the total area of thereflective surface 112 of the base 110. In practice the solid statelight emitter area ratio ρ_(SSL) has a maximum value of 1.0. If thevalue of the solid state light emitter area ratio ρ_(SSL) is smallerthan 0.1, i.e. ρ_(SSL)<0.1, indicating a relatively large reflectivearea of the base 110 with respect to the area of the top surface 106 ofthe solid state light emitter 108, then the factor c should fulfill thecriterion c≧0.2 in order to have a relatively efficient light emittingmodule. If 0.1≦ρ_(SSL)≦0.25, indicating a reflective area of the base110 that is comparable to the area of the top surface 106 of the solidstate light emitter 108, then the factor c should fulfill the criterionc≧0.3 in order to have a relatively efficient light emitting module. Ifρ_(SSL)>0.25, indicating a relatively small reflective area of the base110 with respect to the area of the top surface 106 of the solid statelight emitter 108, then the factor c should fulfill the criterion c≧0.4in order to have a relatively efficient light emitting module. The valueof the factor c in all cases is in practice smaller than 1.0.

Because a substantial amount of light is reflected, scattered or emittedby the luminescent layer 102 in a direction away from the luminescentlayer 102 towards the base 110, it is advantageous to reuse this lightby reflecting the light back to the light exit window 104 to improve theefficiency of the light emitting module 100. The solid state lightemitter reflection coefficient R_SSL can often not be chosen because itis a fixed characteristic of the specific solid state light emitter 108that has to be used in the light emitting module 100. Therefore, inorder to improve the efficiency of the light emitting module 100, it isadvantageous to have the light reflective surface 112 of the base 110which better reflects impinging light than the solid state light emitter108. Further, it has been found that a significant efficiencyimprovement may be obtained if Rbase>R_SSL+c·(1−R_SSL).

The inventors have further found that even more efficient light emittingmodules are achieved if c≧0.4 for 0.0<ρ_(SSL)<0.1, c≧0.6 for0.1≦ρ_(SSL)≦0.25, and c≧0.8 for ρ_(SSL)>0.25. Still more efficient lightemitting modules are achieved if c≧0.6 for 0.0<ρ_(SSL)<0.1 and c≧0.84for 0.1≦ρ_(SSL)≦0.25.

The properties of partially diffusive reflective light are important forachieving an efficient light emitting module according to the efficientand therefore according to the invention the luminescent layer can alsobe replaced by another layer that has partially diffusive reflectiveproperties wherein incident light is partially reflected diffusively andpartially transmitted.

Depending on the application, there are different requirements for lightemitting modules with respect to their lumen output and the size of thelight emitting area of the light emitting module and the solid statelight emitter. For applications where a certain angular distribution oflight intensity is needed, usually beam-shaping optical elements areapplied. To convert the beam profile of a solid state light emitterlight beam, which is usually close to a Lambertian emitter profile, intoa collimated beam it is necessary to keep the initial light emittingsize relatively small. In this case the brightness of the light emittingmodule should be relatively high, which is determined by the lumenoutput and related to the total area of the light emitting surface 106of the solid state light emitter 108, which, for example, can also beincreased by employing more than one solid state light emitter 108. Forthese applications a relatively high solid state light emitter arearatio ρ_(SSL) is needed. An example is a module for a retrofit halogenlamp.

In applications in which there are no strict requirements to brightnesslevels of the light emitting module, the specific beam shape or thetotal emitting area of the solid state light emitter 108, it ispreferred to have a relatively large reflective base surface 112 withrespect to the partially absorbing surface 106 of the solid state lightemitter 108 in order to provide for a more efficient light recycling andhigher efficiency. For these applications a relatively low solid statelight emitter area ratio ρ_(SSL) is preferable. An example is a highlumen package realized in retrofit bulb applications which imposes onlylimited restrictions to the geometry of the light emitting module.

It should be noted that the reflection coefficients are average numbersover a whole surface to which they relate. The light reflective surfaceof the base may comprise, for example, areas which are less reflectivethan other areas. Further, the reflection of light of differentwavelengths and at different angles of incidence may differ. Preferablythe reflection coefficient is averaged over a spectral range and over anangle of incidence distribution, for example, over the spectral range ofdaylight, or over a spectral range which comprises specific quantitiesof the first color range and of the second color range. Measuring areflectivity coefficient is often performed by pointing a collimatedlight beam of the spectral range to the object of which the reflectivityhas to be measured and measuring the amount of reflected light. This istypically done at one or more angles of incidence and the reflectioncoefficient is a weighted average of the obtained reflectioncoefficients in case of different angles of incidence, wherein theweight factor depends on the amounts of light which impinge at thevarious angles of incidence on the object in the light emitting module.

In some cases the solid state light emitter is attached to a substrate,for example, a ceramic or silicon substrate, and the combination of thesubstrate and the solid state light emitter are attached to anothercarrier layer. This carrier layer may for instance be a metal coreprinted circuit board (MCPCB) also called insulated metal substrate(IMS) or a conventional PCB, such as FR4, or another ceramic carrier,such as alumina or aluminiumnitride or a silicon substrate. In suchsituations, the base of the light emitting module is the combination ofthe another carrier layer and the substrate to which the solid statelight emitter is attached. In other words, the base is the combinationof materials and/or layers on which the solid state light emitters areprovided. Consequently, in this specific case, the base reflectioncoefficient is the weighted average of reflection coefficients of thesubstrates and the carrier layer. It is not necessary that thesubstrate, to which the solid state light emitter is attached, or thecarrier substrate is completely flat. Typically there will be metalelectrodes present on the substrates with a physical height, such asconductive copper tracks to supply power to the emitters. Also, theremay be heat spreading layer applied to the surface. Part of thesubstrate of carrier may be locally thicker to achieve an additionalsupport structure, e.g. for clamping the module or attaching collimatorsto the module or to define a rim, e.g. to separate optical functionsfrom electrical functions. Other electrical components may be present onthe substrate or carrier, such as capacitors, temperature sensors likeNTCs, resistors, ESD protection diodes, Zener diodes, varistors, photosensors such as a photodiode, or integrated circuits (ICs). Thesecomponents may likely be placed outside the circumference of the opticalexit window, but in principle could also be placed inside thecircumference of the optical exit window. In the latter case they willcontribute to the average reflectance of the base. These components maybe covered with a reflective layer to minimize optical losses.

FIG. 1b shows another embodiment of a light emitting module 150according to a first aspect of the invention. Light emitting module 150has a similar structure as light emitting module 100, however, aplurality of solid state light emitters 154, 156 are provided which emitlight of the first color range 114 towards the luminescent layer 102.With respect to the light emitting module 150, the solid state emitterlight reflection coefficient R_SSL is defined as the average of thelight reflection coefficients of the plurality of solid state lightemitters 154, 156.

As seen in FIG. 1b , the solid state light emitter area ratio ρ_(SSL),of the light emitting module 150 is larger than such a ratio of thelight emitting module 100, wherein for the calculation of ρ_(SSL), thetotal summed area of the top surfaces 152, 158 of the solid state lightemitters should be substituted for A_SSL. Thus, in the light emittingmodule 150 a relatively larger amount of light impinges on the solidstate light emitters 154, 156, and thus a relatively larger amount oflight is absorbed by the solid state light emitters 154, 156 than in thelight emitting module 100. The light emitting module 150 is an exampleof a light emitting module in which the area ratio ρ_(SSL), is largerthan 0.25 and wherein the value of the factor c should be larger than orequal to 0.4 in order to have a relatively efficient light emittingmodule.

It is to be noted that in other embodiments, the different solid statelight emitters 154, 156 emit different color ranges. Furthermore, theluminescent layer 102 may comprise different luminescent materials eachhaving a different conversion characteristic such that the light whichis transmitted through the light exit window 104 comprises more thanonly the first color range 114 and the second color range 116.

In FIG. 1a and FIG. 1b each one of the solid state light emitters 108,154, 156 has a top surface 106, 152, 158 which is facing towards thelight exit window 104 and the luminescent layer 102. The top surfaces106, 152, 158 are surfaces through which light of the first color range114 is dominantly emitted in the direction of the luminescent layer. Thedistance between the top surfaces 106, 152, 158 of the solid state lightemitters 108, 154, 156 and the surface of the luminescent layer 102 thatfaces the top surfaces 106, 152, 158 is the distance h which is definedas the length of the shortest linear path between the top surfaces 106,152, 158 of the solid state light emitters 108, 154, 156 and the surfaceof the luminescent layer 102 that faces the top surfaces 106, 152, 158.

The inventors have found experimentally that the optical effect of morereflection by the light reflective surface is not the only factor whichcontributes to a higher light output. A gap and a distance h between thesolid state light emitter(s) 108, 154, 156 and the luminescent layer102, also contributes to the efficiency and light output of the lightemitting module. Each one of the top surfaces 106, 152, 158 has alargest linear size d_(SSL), being defined as the largest lineardistance along a line on the top surface 106, 152, 158. If the topsurfaces are circular, the largest linear size d_(SSL), is the length ofthe diameter of the circle. If the top surface has the shape of a squareor a rectangle, the largest linear size d_(SSL), is the length of thediagonal of the square or of the rectangle. The inventors have realizedthat, if the distance h is too small, too much light impinges back onthe solid state light emitters 108, 154, 156 such that too much light isabsorbed by the solid state light emitters 108, 154, 156. And theinventors have further realized that, if the distance h is larger than aspecific value, the amount of light is emitted back to the solid stateemitters 108, 154, 156 compared to the amount of light which is emittedback to the light reflective surface, may be such that no significantefficiency improvement may be obtained in case the distance h is furtherincreased. Furthermore, the inventors have found that the range ofvalues of the distance h which results in a relatively efficient lightemitting module, depends on the solid state light emitter area ratioρ_(SSL). The distance h between the top surfaces 106, 152, 158 and theluminescent layer 102 is preferably in a range that has a minimum valueof 0.3 times the largest linear size d_(SSL) of the top surfaces 106,152, 158 and has a maximum value of 5 times the largest linear sized_(SSL) of the top surfaces 106, 152, 158 for ρ_(SSL)<0.1. For0.1≦ρ_(SSL)≦0.25 the distance h between the top surfaces 106, 152, 158and the luminescent layer 102 is preferably in a range that has aminimum value of 0.15 times the largest linear size d_(SSL) of the topsurfaces 106, 152, 158 and has a maximum value of 3 times the largestlinear size d_(SSL) of the top surfaces 106, 152, 158. For ρ_(SSL)>0.25the distance h between the top surfaces 106, 152, 158 and theluminescent layer 102 is preferably in a range that has a minimum valueof 0.1 times the largest linear size d_(SSL) of the top surfaces 106,152, 158 and has a maximum value of 2 times the largest linear sized_(SSL) of the top surfaces 106, 152, 158.

The light emitting modules 100 and 150 may be even more efficient if inthe above presented formulas and criteria the factor c is larger thanthe values mentioned above. An efficiency increase in the order of 40%may be obtained with respect to a solid state light emitter with aluminescent layer directly on the top surface.

In light emitting module 150 a plurality of light emitters 154, 156 areprovided, and each one of the plurality of light emitters 154, 156 mayhave a different distance to the luminescent layer 102. If the distancesare different, the average of the distances should be in one of theranges that are defined above. If the solid state light emitters 154,156 each have a different shape and/or size of their top surfaces 152,158, the largest linear size d_(SSL) is defined as the average of thelargest linear sizes d_(SSL) of the top surfaces of the plurality ofsolid state light emitters 154, 156.

If there is a gap and a distance h between the solid state lightemitter(s) 108, 154, 156 and the luminescent layer 102, the solid statelight emitter(s) 108, 154, 156 does not become as warm as it would be inthe case that the luminescent layer 102 is positioned on top of, or veryclose to, the solid state light emitter(s) 108, 154, 156. In this case,the luminescent layer 102 is not directly thermally coupled to the solidstate light emitter(s) 108, 154, 156 and provides or receives to alesser extent the heat of the solid state light emitter(s) 108, 154,156. The efficiency of the luminescent material is higher if thetemperature of the luminescent material is kept within acceptablelimits. Further, the efficiency of the solid state light emitter(s) 108,154, 156 is higher if the temperature of the solid state lightemitter(s) 108, 154, 156 is kept within acceptable limits. Thus, thedistance h between the solid state light emitter(s) 108, 154, 156 andthe luminescent layer 102 results in the photothermal effect of a moreefficient luminescent layer 102. Further, the distance h between thesolid state light emitter(s) 108, 154, 156 and the luminescent layer 102results in a more uniform light flux distribution through theluminescent layer 102 instead of a relatively high light flux in a veryspecific area of the luminescent layer 102. Luminescent materials tendto be sensitive to photosaturation, which means that above a certainflux value, the luminescent material converts light at a lowerefficiency. Also some luminescent materials or binders of thesematerials, such as organic phosphors or organic binders, tend to besensitive to photodegradation, which means that above a certain fluxvalue, the luminescent material or the binder starts to degrade whichtypically results in a lowering of efficiency. Thus, by creating adistance h between the solid state light emitter(s) 108, 154, 156 andthe luminescent layer 102 photosaturation of the luminescent materialand photodegradation effects are prevented. Also the distance h aids inachieving a more uniform light output distribution in the exit windowand aids to mix color distributions between the first spectral range(s)and the second spectral range(s). So both the spatial and angular colorhomogeneity is improved. This may be further enhanced with a diffuser ordichroic layer on top of the solid state light emitter or in the lightexit window.

The solid state light emitter(s) 108, 154, 156 may be light emittingdiode(s) (LEDs), organic light emitting diode(s) (OLEDs), or, forexample, laser diode(s), for example vertical-cavity surface-emittinglaser(s) (VCSEL).

FIG. 2a and FIG. 2b present top views of light emitting modules 200, 250according to the first aspect of the invention. The presented top viewsare seen if one faces towards a surface of the base of the lightemitting modules 200, 250 on which the solid state light emitters areprovided via the light exit window. It is to be noted that theluminescent layer is not drawn in FIGS. 2a and 2 b.

In FIG. 2a a light reflective surface 204 of a base and a top surface206 of a solid state light emitter is drawn. Arrow 202 indicates thelargest linear size d_(SSL) of the top surface 206 of the solid statelight emitter. The area of the top surface 206 of the solid stateemitter is L_(w)L_(h). The area of the light reflective surface 204 ofthe base is (B_(W)B_(h)−L_(W)L_(h)), which is the total area of the baseminus the area of the base that is occupied by the solid state lightemitter. Thus, the area of the light reflective surface 204 of the basedoes not include the area of the base that is covered by the solid statelight emitter.

FIG. 2b shows a light reflective surface 254, a first top surface 256 ofa first solid state light emitter, and a second top surface 258 of asecond solid state light emitter. A largest linear distance of therectangular first solid state emitter is indicated by arrow 252. Thearea of the first top surface 256 of the first solid state light emitteris L1_(w)L1_(h). The second top surface 258 of the second solid statelight emitter is circular and its diameter is indicated with arrow 260.The area of the second top surface 258 of the second solid state lightemitter is ¼π(L2_(d))². The area of the light reflective surface 254 ofthe base is in this case (B_(w)B_(h)−L1_(w)L1_(h)−¼π(L2_(d))²).

FIG. 3a presents an embodiment of a light emitting module 300 whichcomprises a cavity 316. The light emitting module 300 comprises a base309 which has a light reflective surface 306 inside the cavity 316. Onthe light reflective surface 306 is provided a solid state light emitter312 which emits light in a first color range towards the light exitwindow. The light exit window is formed by a luminescent layer 308. Inbetween the base 309 and the luminescent layer 308 walls 314 areprovided, in this case four walls 314. The inner surfaces 304 of thewalls 314 are light reflective and have a wall reflection coefficientRwall. The wall reflection coefficient is the ratio between an amount oflight which is reflected by the light reflective surface 304 of thewalls 314 and an amount of light which impinges on the light reflectivesurface 304 of the walls 314. The solid state light emitter 312 has asolid state light emitter reflection coefficient R_SSL. The lightreflective surface 306 of the base 309 has a base reflection coefficientRbase. The definition of the base and the solid state light emitterreflection coefficient are given in the description of FIGS. 1a and 1 b.

The walls 314 may consist of various materials. The wall material mayprovide a high reflectivity such as when using a scattering ceramic suchas reflective alumina, zirconia or other ceramics, a scattering glass, ascattering pigmented polymer, such as white polyamide; or scatteringfluorpolymers, like Spectralon or a scattering silicone resin. The walls314 may also consist of a metal material such as aluminium or silver.The metal may be a metal foil or film, such a highly reflectivecommercial metal mirrors with the trade name of Alanod.

The wall material may also be of low reflectivity and covered with areflective layer. In this case the wall may comprise another materiallike a thermally conductive polymer, such as a carbon filled plastic,e.g. a polyamide, or metallic materials like copper, nickel, stainlesssteel or ceramic materials such as aluminium nitride (AlN). Thesematerials typically have a high thermal conductivity which isbeneficial, e.g. copper=˜400 W/mK, AlN=˜140 W/mK. The reflective layermay be a coating, a film or a thin layer. The reflective layer may forexample be a casted, dipped, dispensed or sprayed layer of whitesilicone or a white solgel, for example an alkylsilicate based material,pigmented with scattering particles such as TiO2 or ZrO2. Or thereflective layer may for example be a thin metal coating such asprotected silver or aluminium which may be evaporated or sputtered onthe wall material. The walls 314 may come in a variety of shapes suchas, for example, circular such as a ring, cylindrical, squared ortriangular. The wall may contain surface structures such as fins inorder to facility cooling.

The wall material may also consist of a thin film layer, such as thereflector coating or film only. In this case the wall reflector maycover the edges of a solid material present between the base and theluminescent material, such as the circumference of a glass or ceramicsubstrate.

The wall may be a diffuse reflector or a specular reflector. In someembodiments a specular reflecting wall shows a better performance than adiffuse reflecting wall, and in other embodiments a diffuse reflectingwall shows a better performance than a specular reflecting wall.

Further, the base 306 and the walls 314 may comprise a heat conductingmaterial. The luminescent layer 308 preferably is thermally connected tothe walls 314 at the edges of the luminescent layer 308. For example, aheat conducting paste or heat conducting adhesive may be used to connectthe luminescent layer 308 to the walls 314. At the base 306 an interfaceto a heat sink (not shown) may be provided. The base 306 may be part ofthe heat sink, or the base 306 may constitute the heat sink. The solidstate light emitter 312 is provided within the cavity 316 and is appliedto the light reflective base 306. The contact between the solid statelight emitter 312 and the light reflective base 306 is such that thesolid state light emitter 312 is thermally coupled to the base 306. Thesolid state light emitter 312 may be soldered, or glued with a heatconducting adhesive, for example a metal particle filled adhesive, tothe light reflective base 306. The base 306 of the cavity 316 and/or thewalls 314 may contain thermal vias to further improve heat transfer. Forexample, the base 306 may be made of an aluminum oxide ceramic thatcontains through holes which are metalized with copper. The copper has ahigher thermal conductivity (approx. 400 W/mK compared to the aluminumoxide (20-30 W/mK). The solid state light emitter 312 may also beconnected with electrical vias through the base 306 of the cavity 316 toa power source. The electrical vias may also conduct heat.

The luminescent layer may comprise phosphors for converting light in thefirst color range into light of the second color range. The second colorrange is preferable different from the first color range—however, theranges may partially overlap. The phosphor may be a yellow phosphor likeYAG:Ce, LuAG:Ce or LuYAG:Ce, for partial conversion of blue lightgenerated by the solid state light emitter to yellow light such that acombined substantial white light emission may be obtained. In anotherembodiment the phosphor may be a full conversion phosphor like BSSNE:Euor ECAS:Eu for fully converting blue light to amber light or red light,respectively. The luminescent layer may comprise a combination ofphosphors, e.g. YAG:Ce and ECAS:Eu to obtain a warmer white lightemission.

The conversion of the light of the first color range into light of thesecond color range has a high efficiency, however, some light isabsorbed and converted into heat. Especially with high power solid statelight emitters the amount of absorbed energy may be relatively high. Theefficiency of the luminescent layer may degrade if the luminescent layer308 becomes too hot, for example more than 200° C. Further, theluminescent layer may comprise materials which degrade at hightemperatures such that their light emission characteristics degrade aswell. In the light emitting module the generated heat is transferred viathe walls and the base towards a heat sink. As such the luminescentlayer does not become too hot.

The luminescent layer may be a ceramic phosphor which is manufactured toa fused macroscopic body via sintering of powder particles of phosphoror from precursor powders that form the phosphor in a reactive sinteringprocess. Such a ceramic phosphor is produced in plates and these platesare mechanically diced to give a proper size matching the light exitwindow. It is to be noted that a single sheet of luminescent material,like a sheet of the ceramic phosphor, may cover a plurality ofneighboring cavities.

A ceramic phosphor is a relative good thermal conductor. The thermalconductivity depends on the type of ceramic phosphor and the residualporosity. As an example typical thermal conductivity for a ceramic Cedoped YAG phosphor is 9-13 W/mK at room temperature. A typical thermalconductivity of a powder phosphor layer in a binder resin such as asilicone or organic polymer is dominated by the binder with a thermalconductivity of about 0.15-0.3 W/mK. The ceramic phosphor layer may bearound 10-300 micron in thickness, typically about 100 micron, and istherefore rigid, self-supporting, hence no additional supportingsubstrate is need for the luminescent layer.

The luminescent layer may also be a substrate of glass on which a layerof a translucent resin comprising phosphor particles is deposited. Forexample, a powder with phosphor particles which are dispersed in abinder, typically a silicone resin. Preferably however, the binder is abetter heat conducting material such as a glass or sol-gel derivedsilicate or alkylsilicate with a typical heat conductivity around 1W/mK. The luminescent layer may also be sandwiched in between two otherlayers, for example the luminescent layer is applied on a glass layerand on top of the luminescent layer a glass layer is applied, whichimproves the spreading of heat. Examples of other layer combinations areceramic layer—luminescent layer—glass layer and ceramiclayer—luminescent layer—ceramic layer.

In an embodiment, an additional layer is placed on top of theluminescent layer which acts as a diffuser such that the light emittingmodule 300 emits light in a plurality of output directions with improvedangular color uniformity. The luminescent layer will convert lighttravelling more or less perpendicular through the luminescent layer lessthan light travelling at large angles with the normal. In case a partialconverted luminescent layer is used this induces more light (typicallyof blue color) to be emitted near the normal angle than at large angles.This leads to unacceptable color variations with angle. The diffuserscrambles the light prior to emission towards the ambient to improve thecolor-over-angle uniformity. The diffuser is preferably dominantlyforward scattering.

Alternatively a dichroic or interference layer may be present on top ofthe luminescent layer to correct the color-over-angle errors in thelight emitted through the luminescent layer. The dichroic layer consistsof a multitude of thin layers with alternative higher and lowerrefractive indices with which the light interferes. The opticalcharacteristics of the dichroic layer are such that blue light isreflected more near the normal, and less or not at larger angles in agradual way. The excess of a blue solid state light emitter near thenormal through the phosphor is then compensated by a higherbackreflection by the dichroic layer. The backreflected blue light willpartially excite the phosphor and be color converted and is partially berecycled in the cavity. The dichroic layer may be present as a thin filmon a carrier substrate, such as a glass, and connected to the phosphor.The connection may be made using an adhesive.

Alternatively the phosphor may be present as a coating on the samesubstrate as the dichroic layer at the opposite side. The carriersubstrate of the dichroic layer may be a heat conducting transparentsubstrate such as a ceramic.

Light which is reflected or scattered by the luminescent layer and whichis emitted by the luminescent layer is also reflected towards the walls314 and is reflected by the light reflective surfaces 304 of the walls314. As such light, which is not immediately transmitted through thelight exit window into the ambient, is reflected via the lightreflective surfaces 304 of the walls 314 and/or the light reflectivesurface 306 of the base 309. Thus, the light which is not immediatelytransmitted into the ambient is recycled more efficiently andcontributes to an efficient light emitting module. In this case aneffective reflection coefficient Reff is defined as a weighted averageof the base and the wall reflection coefficient, or, in other words, theeffective reflectivity is a weighted average of the base and the wallreflection coefficients. The effective reflection coefficient Reff maybe defined as:

$\begin{matrix}{{Reff} = \frac{{{Rbase}*{Abase}} + {{Rwall}*{Awall}}}{{Abase} + {Awall}}} & (1)\end{matrix}$wherein the base reflection coefficient Rbase is the reflectioncoefficient of the light reflective surface 306 of the base 309, thewall reflection coefficient Rwall is the reflection coefficient of thelight reflective surfaces 304 of the walls 314, Abase is the total areaof the reflective surface 306 of the base 309 and Awall is the totalarea of the reflective surfaces 304 of the walls 314.

In this embodiment, the value of the effective reflection coefficientReff should be at least larger than the value of the solid state lightemitter reflection coefficient R_SSL. Preferably, the value of theeffective reflection coefficient Reff should be at least larger than thevalue of the solid state light emitter reflection coefficient R_SSL plusa factor c times the difference between 1 and the solid state lightemitter reflection coefficient R_SSL. Thus, Reff>R_SSL+c(1−R_SSL). Thefactor c is, similar to the embodiments described in FIGS. 1a and 1b ,dependent on the solid state light emitter area ratio ρ_(SSL), which inthis case is defined as:

$\begin{matrix}{\rho_{SSL} = \frac{A\_ SSL}{{Abase} + {Awall}}} & (2)\end{matrix}$Thus, as compared to the embodiments of FIGS. 1a and 1b , in this casethe area of the reflective surfaces 304 of the walls 314 is also takeninto account, i.e. the total reflective area now comprises the base andthe wall reflective area. If ρ_(SSL)<0.1, indicating a relatively largereflective area of the base 309 and walls 314 with respect to the areaof the top surface of the solid state light emitter 312, then the valueof the factor c should be larger than or equal to 0.2 in order to have arelatively efficient light emitting module. If 0.1≦ρ_(SSL)≦0.25,indicating a reflective area of the base 309 and walls 314 that iscomparable to the area of the top surface of the solid state lightemitter 312, then the value of the factor c should be larger than orequal to 0.3 in order to have a relatively efficient light emittingmodule. If ρ_(SSL)>0.25, indicating a relatively small reflective areaof the base 309 and walls 314 with respect to the area of the topsurface of the solid state light emitter 312, then the value of thefactor c should be larger than or equal to 0.4 in order to have arelatively efficient light emitting module. The value of the factor c inboth cases is in practice smaller than 1.0.

FIG. 3b shows another embodiment of a light emitting module 350according to the first aspect of the invention. The light emittingmodule 350 is similar to light emitting module 300 of FIG. 3a . However,there are some minor differences. Light emitting module 350 has acircular base 358 with a light reflective surface 354 which is facedtowards a cavity. The cavity is enclosed by the base 358, a cylindricalwall 362 and a luminescent layer 352. A surface of the cylindrical wall362 which faces towards cavity is a light reflective wall surface 356.On the light reflective surface 354 of the base 358 a plurality of solidstate light emitters are provided which emit light of a first colorrange towards the light exit window of the cavity. The light exit windowof the cavity is formed by a luminescent layer 352 which comprisesluminescent material for converting a part of the light of the firstcolor range towards light of a second color range.

Also for this embodiment ρ_(SSL) is defined as the ratio of the totalsummed area of the top surfaces of the solid state light emitters 360and the area of the reflective surface 354 of the base 358. The samecriteria and ranges apply as are described with reference to FIG. 3 a.

A cross-section of the light emitting module 300 of FIG. 3a along lineA-A′ is presented in FIG. 4a . The light exit window is indicated with402. The light exit window 402 is a portion of the luminescent layer 308because a part of the luminescent layer 308 is arranged on top of thewalls 404, 314 which have a certain thickness. Alternatively, there maybe a recess in the wall edge to which the luminescent layer 308 may befitted as a support of the luminescent layer 308. An adhesive may beused to attach the luminescent layer 308 to the top of the wall or intothe recess in the wall. In the case that a recess is used to attach theluminescent layer 308, there is an additional benefit of achievingthermal contact of the side face of the luminescent layer 308 to thewall.

Thus, the value of the effective reflection coefficient Reff should beat least larger than the value of the solid state light emitterreflection coefficient R_SSL. Preferably, the value of the effectivereflection coefficient Reff should be at least larger than the value ofthe solid state light emitter reflection coefficient R_SSL plus a factorc times the difference between 1 and the solid state light emitterreflection coefficient R_SSL. The factor c is, similar to theembodiments described with reference to FIGS. 1a and 1b , dependent onthe solid state light emitter area ratio ρ_(SSL), which in thisembodiment also includes the area of reflective surface 356 of the walls362. ρ_(SSL)<0.1, indicating a relatively large reflective area of thebase 309 and walls 404, 314 with respect to the area of the top surfaceof the solid state light emitter 312, then the value of the factor cshould be larger than or equal to 0.2 in order to have a relativelyefficient light emitting module. If 0.1≦ρ_(SSL)≦0.25, indicating areflective area of the base 309 and walls 404, 314 that is comparable tothe area of the top surface of the solid state light emitter 312, thenthe value of the factor c should be larger than or equal to 0.3 in orderto have a relatively efficient light emitting module. If ρ_(SSL)>0.25,indicating a relatively small reflective area of the base 309 and walls404, 314 with respect to the area of the top surface of the solid statelight emitter 312, then the value of the factor c should be larger thanor equal to 0.4 in order to have a relatively efficient light emittingmodule. The value of the factor c in both cases is in practice smallerthan 1.0.

Furthermore, the inventors have found that the distance h between thetop surface 412 of the solid state light emitter 312 and the luminescentlayer 308 preferably is in a range that has a minimum value of 0.3 timesthe largest linear size d_(SSL) of the top surface 412 and has a maximumvalue of 5 times the largest linear size d_(SSL) of the top surface 308for values of the solid state light emitter area ratio ρ_(SSL) smallerthan 0.1. For 0.1≦ρ_(SSL)≦0.25, the distance h between the top surface308 and the luminescent layer 102 is preferably in a range that has aminimum value of 0.15 times the largest linear size d_(SSL) of the topsurface 308 and has a maximum value of 3 times the largest linear sized_(SSL) of the top surface 308. For ρ_(SSL)>0.25 the distance h betweenthe top surface 308 and the luminescent layer 102 is preferably in arange that has a minimum value of 0.1 times the largest linear sized_(SSL) of the top surface 308 and has a maximum value of 2 times thelargest linear size d_(SSL) of the top surface 308.

It is noted that, if the solid state light emitter 312 fulfills theabove criteria, the light emitting module 300 is a relatively efficientlight emitting module. Absorption by the solid state light emittercontributes significantly to the inefficiency, while all otherdistances, sizes and reflection coefficients are optimized for maximumlight output. The light emitting module 300 may be even more efficientif in the above presented formulas the factor c is larger than thevalues mentioned above. An efficiency increase in the order of 40% maybe obtained with respect to a light emitting module with a luminescentlayer directly on the top surface of the solid state light emitter.

The luminescent layer 308 is placed on a top edge of the walls 404, 314and as such the luminescent layer 308 is thermally coupled to the walls404, 314. The luminescent layer 308 becomes warm because of theabsorption of energy by the luminescent material while it converts lightof the first color range towards light of the second color range. Thethermal coupling between the luminescent layer 308 and the walls 404,314 allows the walls 404, 314 to conduct the heat of the luminescentlayer towards the base 309, which may comprise an interface for couplingthe base 309 to a heat sink. This mechanism provides an effective heatmanagement of the light emitting module 300 and prevents that theluminescent layer 308 becomes too warm, which enhances the efficiencyand the lifetime of the luminescent material. Further, the cavity 316may be filled with a substantially optically transparent material. Ifthe whole cavity is filled with the transparent material, thetransparent material is also thermally coupled to the luminescent layer308 and may conduct heat away from the luminescent layer towards thewalls 404, 314 and the base 309 in a much more efficient way than whenan air gap is used. As will be discussed in the context of FIG. 5a thetransparent material has further advantages such as the increase oflight outcoupling from the solid state light emitter 412.

The substantially transparent material is typically a solid material,such as a solidified or cured silicone resin with a thermal conductivityof 0.2 to 0.3 W/mK. Many types of such materials exist, ranging fromhard silicone resins, to soft silicone resins, to flexible elasticsilicone resins or gel type of resins. Other materials may include epoxyresins, many types of optically transparent polymers known to thoseskilled in the art. In other embodiments, wide range of glass type ofmaterials may be used, such as sodalime glass of about 1.0 W/mK thermalconductivity or borosilicate glass or a fused silica glass of about 1.3W/mK. Also, ceramic materials may be used such as translucentpolycrystalline alumina substrates of about 30 W/mK, sapphire substratesof 42 W/mK, AlON of 9.5 W/mK, spinel of 15 W/mK or YAG of 7 W/mK thermalconductivity. Combinations of such materials may also be used. Forexample, solid glass or ceramic substrates may be bonded to the emittersand/or the base. Also, sintered translucent polycrystalline alumina maybe used as the substantially transparent material, wherein the grainsize is preferably larger than 44 um or preferably smaller than 1 um inorder to obtain relatively high forward light transmission. The totalforward transmission of light is larger than 84% for 1 mm thick materialand a grain size that is smaller than 1 um. The total forwardtransmission of light is larger than 82% for 1 mm thick material and agrain size that is larger than 44 um. The polycrystalline alumina can bemade with, for example, a ceramic powder processing technology in whichAl₂O₃ powder is shaped, for example by powder pressing, slip casting,injection molding, and pre-sintered and end-sintered. A relatively largegrain size, i.e. larger than 44 um, may be achieved by applying analumina powder with a relatively large grain size, by applying a longersinter time and/or a higher sinter temperature, using less grain growthsuppressing MgO doping (<300 ppm) and/or apply grain growth stimulatingdopes or a combination of one or more of the above methods. Preferably,the grain size is smaller than 120 um to prevent micro-cracking of thepolycrystalline alumina. In this way the excellent thermal properties ofthis material, because the thermal conductivity is about 30 W/mK, arecombined with a relatively high translucency.

Optionally the optical and thermal contact is achieved with the emittersurface such as to extract more light from the emitter and an air gap isstill present between the solid material and the base. This may help tospread out the light more effectively by lightguiding in the solidmaterial to enhance light uniformity. For optimal thermal contact, thesolid substrates may also be attached to the base, for instance using anadhesive. The solid substrate performs the function of a heat spreadinglayer and thermal interface material in case it is also coupled to theluminescent layer. The solid material may also be present on theemitter, such as a piece of sapphire or silicon carbide SiC, which maybe the growth substrate on to which the emitter die was formed.Furthermore a dome shape or lens shape optical body may be present onthe die, typically of a size at least 2 times larger than the longestlinear size, which may, for example, be from a silicone resin of a glassmaterial. The dome or lens shaped body may be covered with anothertransparent material.

The substantially transparent material preferably has a relatively highrefractive index if in optical contact to the emitter die. As typicalsolid state light emitters, like GaN or InGaN or AlInGaN, have a highrefractive index of about 2.4, a high refractive index contact to thedie extracts more light from the die by reducing total internalreflection in the solid state light emitter chip. Most transparentmaterials come with a refractive index ranging from 1.4 to 1.6,typically 1.5. Some examples of high refractive index materials suitablefor attaching to the emitter are high refractive index glasses, likeLaSFN9, or ceramic materials like sapphire (n˜1.77), alumina (n˜1.77),YAG (n˜1.86), zirconia (n˜2.2) or silicon carbide (SiC, n˜2.6). A highrefractive index optical bond may be used to attach the substrates, suchas a high index glass or a high index resin. The high index resin mayconsist of a low index binder filled with high refractive indexnano-particles, such as silicone resin filled with nano-TiO₂ particlessmaller than 100 nm in diameter or other high index nano-particles suchas ZrO2 or titanates such as BaTiO3, SrTiO3. In some types of emitterdies the typical growth substrates such as sapphire and silicon carbidemay still be present on the die. Preferably these dies are covered inthis case with a high refractive index material, such as describedabove.

Alternatively also liquid materials may be used, such as silicone oils(n˜1.4) or mineral oils (n˜1.5) or a wide variety of liquids, such asaliphatic or aromatic hydrocarbons, or liquids of high refractive index,known to those skilled in the art. When a liquid is used a tight sealingaround the edges of the exit window is preferred to prevent leakage fromthe light emitting module. The liquid may serve the purpose of coolingthe luminescent layer by convective flow and/or by being pumped around.

FIG. 4b shows a cross-section of another embodiment of thelight-emitting module of FIG. 3a . A light emitting module 450 comprisesa housing 455, a cavity 460, a luminescent layer 465, an interface 470to a heat sink 480 and a light exit window 472. The housing 455 in thiscase comprises both a base and walls with a light reflective basesurface 462 and light reflective wall surfaces 466, 468. A specific typeof solid state light emitter 482 is shown which is connected to theelectrical power by means of two wires 492. LEDs often have bond wires492 that are connected to the solid state light emitter 482 at a topsurface 483 of the solid state light emitter 482. The top surface 483 isa surface of the solid state light emitter 482 which is closest to theluminescent layer 465 and where the light is emitted into the cavity460. In some embodiments there are two electrical wire contacts at thetop surface 483, and in other embodiments there is one electrical wirecontact at the top surface 483 and one electrical contact at a bottomsurface of the solid state light emitter 482 to the base.

As seen in FIG. 4b the interface 470 to the heat sink 480 is provided atthe back side of the light emitting module 450. It is to be noted thatthe back side is substantially opposite the side where the luminescentlayer 465 is present and that a part of the housing which forms the backside also forms the base of the cavity 460. As seen in FIG. 4b the solidstate light emitter 482 is applied to the light reflective base 462 ofthe cavity 460. The contact between the solid state light emitter 482and the housing 455 is such that a good thermal coupling is obtainedbetween the solid state light emitter 482 to the housing 455 and as suchbetween the solid state light emitter 482 and the heat sink 480.

Alternatively, the solid state light emitter 482 may be mounted in athrough hole in the light reflect base such that light is emitted intothe cavity 460 and such that the solid state light emitter 482 has agood thermal contact with the housing 455.

A wire-bond top connection 492 is a wire which is electrically connectedto an electrical contact area at the top surface 483 of a LED 482 whichis usually metalized and the wire provides electrical energy to the LED482. The top surface 483 of the LED 482 is often the light emittingsurface of the LED 482 as well. The light emitting surface of the LED482 is defined as the non-obstructed emissive surface area of the LED482 where the light generated by the LED 482 is emitted into the cavity460. In this embodiment the top surface 483 of the LED 482 is thesurface which faces towards the luminescent layer 465.

Using a luminescent layer 465, which is implemented as a ceramicphosphor, or which is implemented as a phosphor layer deposited on forexample a glass substrate in combination with the LED 482 with awire-bond top connection 492 has proved to be difficult. The wires 492obstruct the direct provision of such a ceramic phosphor layer on top ofthe light emitting surface. A solution may be to drill precision holesin the ceramic phosphor through which the wire is led, which is arelative expensive process. However, it is difficult to prevent lightleakage via the precisions holes along the wire. This results in areduced color control. Especially when the luminescent layer 465 has toconvert most of the light of in the first color range, the light leakageresults in an unacceptable reduced color saturation. Further, the holeswould typically be drilled with laser ablation which comes with the riskof damaging the phosphor near the drilled holes such that the ablationby-products absorb light and a part of the phosphor is deactivated.

Typical ceramic phosphors, like YAG:Ce and amber colored bariumstrontium silicon nitride (BSSNE:Eu) have a refractive index of about1.86 and 2, respectively. Thus, a transparent resin with a refractiveindex higher than 1.4 may provide a relatively good optical couplingbetween these specific LEDs and the discussed specific ceramicphosphors. Extra scattering centers, like scattering particles, may beincorporated preferably with forward scattering characteristics.

The embodiment provides an effective and efficient solution forconverting light of a LED 482 with one or more wire bond top connections492 into another color. The cavity 460 provides space for the wires 492,and because of the reflections of the light inside the cavity no shadowof the wires 492 is visible at the light exit window 472. It is to benoted that the cavity 460 of the embodiment is relative large withrespect to the size of the light emitting module 450 and as such lessshadows of wires may be available compared to the known light emittingmodules in which the cavity is relative small.

The use of the wire-bond top-connection 492 together with a transparentresin 498, which is arranged between the LED 482 and the luminescentlayer 465, is advantageous. The transparent resin 498 may be injectedinto the cavity 460 after assembling the LED 482 to the housing 455.During injection the transparent resin 498 is in a liquid state and mayflow towards each corner of the cavity. The wires 492 are not anobstacle for the injected transparent resins and as such a good contactmay be made between the transparent resin 498 and the whole top surface483 of the LED 482. Thus, the transparent resin 498 increases theoutcoupling of light from the LED 482. Further, if the transparent resin498 is hardened the wire-bound top connections 492 are fastened by theresin 498 and is less sensitive to damage, for example, if the lightemitting module 450 is subject to vibrations like for example inautomotive applications.

FIG. 5a presents several alternative embodiments of the light emittingmodule according to the first aspect of the invention. Light emittingmodule 500, depicted in FIG. 5a (i), comprises a base 518, a pluralityof LEDs 514 provided on substrates 516, walls 510, a first luminescentlayer 506 and a second luminescent layer 504 provided on an edge of thewalls and forming a light exit window. The LEDs 514 emit light of afirst color range and all LEDs 514 have the same size with a longestlinear size d. The first luminescent layer 506 comprises luminescentmaterial for converting light of the first color range into light of asecond color range. The second luminescent layer 504 comprises anotherluminescent material for converting light of the first color range intolight of a third color range or for converting light of the second colorrange into light of the third color range. The walls 510, the base 518and first luminescent layer 506 enclose a cavity which is filled with atransparent material 502. Thus, the transparent material is interposedbetween the LEDs 514 and the first luminescent layer 506. Thetransparent material is optically coupled to the LEDs 514 and opticallyand thermally coupled to the first luminescent layer 506. The distancebetween the light emitters and the first luminescent layer 506 isindicated with h. The surfaces of the walls 510 which face towards thecavity are provided with a light reflective coating 508. The spacesbetween the LEDs 514 and the light transmitting material 502 are filledwith a light reflective material 512, thereby covering the base 518 andthe substrates 516. The light reflective surface is formed by thesurface of the light reflective material 512 which is interposed betweenthe LEDs 514. The light reflective material has a base reflectioncoefficient Rbase. The LED dies have a reflection coefficient R_SSL. Thelight reflective coating 508 has a wall reflection coefficient Rwall.The parameters of the light emitting module 500 relate to each otheraccording to the same criteria as described in the previous embodimentswith reference to FIGS. 1a, 1b, 3a, 3b and 4a , wherein the area of thetop surface of the solid state light emitter A_SSL is in this embodimentcalculated as the summed total area of the top surfaces of the pluralityof LEDs 514.

Instead of a light reflective coating also a light reflective foil orfilm may be used that can be attached to or transferred to the base andor walls. An adhesive may be used for the attachment, such as a pressuresensitive adhesive. The reflective coating layer may be a dielectriclayer as is typically used in an MCPCB carrier to isolate the surfaceelectrodes from the metal carrier or a solder mask typically used in anMCPCB or PCB carrier to screen-off the surface electrodes. As thesubstrate 516 is covered with a reflective layer and is hence opticallyscreened off, it may consist of a material with poor reflectivity suchas aluminiumnitride (AlN). AlN has the advantage of having a very highthermal conductivity of about 140 W/mK. Hence, optical functions can bescreened off from thermal functions by the use of a reflective coatingor foil allowing individual optimization of both functions which isadvantageous.

The light reflective coating or film may consist of a diffuselyreflecting material, such as a white coating consisting of a binderfilled with a scattering pigment or various scattering pigments.Suitable binders are silicone materials or silicate materials oralkylsilicate materials or epoxy materials or polyimide materials orfluorpolymers or polyamides or polyurethanes or other polymericmaterials. The coating may also consist of highly reflectiveBariumSulphate (BaSO4) based material. Examples of scattering pigmentsare TiO2 pigments, ZrO2 pigments, Al2O3 pigments, but many otherscattering particles or pores may be used as well, known to thoseskilled in the art. The reflective coating or film may also consist ofmetal layers, such as aluminium or silver. The metal may be a metal foilor film, such a highly reflective commercial metal mirrors with thetrade name of Alanod. The thin metal layer may be evaporated orsputtered on the wall material. The metal foil may be used as in insertattached/bonded/soldered to the base. The metal layer may be coveredwith a white coating layer, for example a white silicone or whitealkylsilicate layer, such as. a pigmented methylsilicate. A ceramicreflector layer may also be used on the base or the walls, for example ascattering alumina layer, typically porous, or other reflective ceramicmaterial.

Light emitting module 520, depicted in FIG. 5a (ii), is similar to lightemitting module 500, however, the walls 522 themselves are of a lightreflective material, and as such no additional coating is applied to thewalls 522. Further, only one luminescent layer 506 is applied. Thesubstrates 524 on which the LEDs 514 are provided are also of a lightreflective material, and as such only the spaces between the substrates524 are filled with light reflective particles 512.

Light emitting module 530, depicted in FIG. 5a (iii), is anothervariation in which so-called domed LEDs 514 are used. The LEDs 514 areprovided on a substrate 516 and domes of a light transmitting material502 are placed on top of the LEDs. The dome of the light transmittingmaterial 502 is optically coupled to the die of the LED. Further, thecavity is filled with a further light transmitting material 532. Thefurther light transmitting material 532 is optically coupled to thedomes of the light transmitting material 502 and is optically coupled tothe first luminescent layer 506. This facilitates thermal transfer ofheat generated in the luminescent layer towards the base and the heatsink to which the base is typically attached.

Light emitting module 540, depicted in FIG. 5a (iv), is similar to lightemitting module 500, however, the walls 542 are tilted with respect to anormal axis to the base 518. The walls 542 are tilted in a way such thatlight which impinges on the tilted walls 542 is reflected towards thefirst luminescent layer 506 instead of a direction towards the base 518.The tilted walls 542 direct the light reflected on the walls 542 towardsthe luminescent layer 506 and prevent that light rays are reflected manytimes between the walls 542 and base, which prevents unnecessary lightabsorption, namely, every reflection is not perfect and at everyreflection a small amount of light is absorbed.

Light emitting module 550, depicted in FIG. 5a (v), is a variant oflight emitting module 540. The walls 552 of light emitting module 550are curved in a way such that more light, which impinges on the curvedwalls 552, is reflected towards the first luminescent layer 506 and thustowards the light exit window. Furthermore, the substrate surfaces 516are not coated but the spacing 512 between the substrates is coated witha reflective material. The substrate 516 may consist of a reflectivematerial, such as a scattering ceramic, such as alumina that includesscattering pores and/or scattering particles, such as zirconiaparticles. Thus the reflectance of the light reflective surface of thebase 518 is an average of the reflectance of the substrate 516 and thespacing 512 weighted over the area.

Light emitting module 560, depicted in FIG. 5a (vi), is anothervariation which does not comprise the second luminescent layer 504. Thecavity is filled with a substantially transparent material 562 and hasat the light exit side of the light emitting module a curved surface.The first luminescent layer 506 is provided on top of the transparentmaterial 562. As shown, the distances between the LEDs 514 and the firstluminescent layer 506 differ. Two LEDs are positioned at a distance h1from the first luminescent layer 506, and two LEDs are positioned at adistance h2 form the first luminescent layer 506. The value of thedistance h between the top surface of the LEDs 514 in this embodimentshould be calculated as the average distance: h=(h1+h2)/2. In the casethat three or more LEDs are applied in the light emitting module, theaverage distance formula is adapted accordingly.

In yet another embodiment, which is not shown, the solid state emitterdies are bonded directly to the carrier board without the additionalintermediate substrate. This further reduces thermal resistance betweenthe die and the board and the die and the heat sink to which the boardis typically attached. Wire bonds may be present to electrically contactthe top of the LED die.

FIG. 5b presents four alternative light emitting modules 570, 580, 590,595. Light emitting module 570, depicted in FIG. 5b (i), is similar tolight emitting module 520 and has inside the cavity an additionalluminescent layer 572. Thus, for example, a layer with another type ofluminescent material may be applied to the light reflective walls 522and the light reflective surface of the base 518, which is differentfrom the luminescent material as applied in the first luminescent layer506. This another luminescent material converts light of the first colorrange towards light of the third color range. Alternatively, the sameluminescent material as used in the first luminescent layer may beapplied to the light reflective walls 522 and the light reflectivesurface of the base 518. Not all light which impinges on the additionalluminescent layer 527 is converted, and some light is emitted towardsthe light reflective walls 522 and the light reflective surface of thebase 518 and is subsequently reflected back towards the cavity and thustowards the light exit window. For example, this may be used to addadditional red light to a white emission to achieve a warm whiteemission.

Light emitting module 580, depicted in FIG. 5b (ii), is similar to lightemitting module 500. A first difference is that only a singleluminescent layer 506 is provided at the light exit window. Duringmanufacturing the luminescent layer 506 is applied to a transparentsubstrate 582, which is for example glass. The substrate 582 with theluminescent layer 506 is cut into pieces, for example with a saw, ordrilled and a piece of the substrate 582 with the luminescent layer 506is provided on the walls 510 of the light emitting module 580.

Light emitting module 590, depicted in FIG. 5b (iii), is similar tolight emitting module 580, however, the cavity is not filled with asubstantially transparent material, but with a piece of the transparentsubstrate 582 with the luminescent layer 506. The piece is bonded with,for example, a transparent resin 592 to the light reflective wallsurfaces and the light reflective surface of the base 518. Thetransparent substrate 582 is, for example, 2 mm thick and provides assuch a height difference between the top surfaces of the LEDs 514 andthe luminescent layer 506 of about 2 mm. On top of the device a whitesilicone rim may be applied at the circumference of the luminescentlayer 506 to prevent escape of direct light of the light emitted by theLEDs 514 (e.g. blue) (not shown).

Light emitting module 595, depicted in FIG. 5b (iv), is similar to lightemitting module 520. However, other types of LEDs are used. The base 598is a metal core PCB (MCPCB). LEDs without a relatively large substratemay be mounted directly on the MCPCB. LEDs which are suitable for suchapplications are LEDs which are manufactured with the so-called CSP orCOB technologies. COB refers to chip-on-board wherein the LED chip issoldered directly on the MCPCB. CSP refers to Chip Scale Packages wherea carrier is provided to the wafer on which the LED is manufactured, andthe wafer is diced to obtain CSP LEDs. Such CSP LEDs are presented inlight emitting module 595. In CSP LEDs the carrier 597 has the same sizeas the LED chip 596. The side surfaces of the CSP may be reflective andthe surface of the PCB may be reflective such that no additional (thick)base reflector layer may be needed.

In FIG. 6 other schematically drawn cross-cuts of embodiments of a lightemitting module 600, 620, 630, 640, 650, 660 are presented. The lightemitting modules 600, 620, 630, 640, 650, 660 do not have walls betweena luminescent layer 604, 622, 632, 642, 652, 662 and the base, but theyhave the luminescent layer 604, 622, 632, 642, 652, 662 of which theedge touches the light reflective surface or base 610, 664. Theluminescent layer 604, 622, 632, 642, 652, 662 as a whole forms thewhole light exit window of the light emitting modules 600, 620, 630,640, 650, 660. The light emitting modules 600, 620, 630, 640, 650, 660do not only emit light in a direction substantially parallel to a normalaxis to the base 610, 664, but also emit light in various light emissionangles with respect to the normal axis of the base. In FIG. 6(ii) aschematical cross-section of light emitting module 620 an edge 624 ofthe luminescent layer 622 is shown. As seen the edge 624 is in contactwith the light reflective surface of the base 610 and the luminescentlayer 622 may extend on the surface of the base.

The light emitting module 600, depicted in FIG. 6(i), comprises a base610, on which substrates 608 with LEDs 606 are provided. The substrates608 and LEDs 606 are surrounded by a light reflective material 612 whichforms a light reflective surface. The light emitting top surfaces of theLEDs 606 are optically coupled to a transparent material 602 which isalso in contact with the luminescent layer 604. Light emitting modules620, 630, 640 have luminescent layers 622, 632, 642 of another shape andare depicted in FIGS. 6(ii), 6 (iii) and 6(iv), respectively.

Light emitting module 650, depicted in FIG. 6(v), has a base 610, onwhich a single chip-scale packaged LED 656 is provided. Often theabbreviation CSP-LED is used for the chip-scaled packaged LED 656—such achip-scaled packaged LED 656 does not comprise an extra substrate asshown in previous embodiments. Around the LED 656 a light reflectivematerial 612 is applied which creates a light reflective surface facingtowards the luminescent layer 652. On top of the LED 656 and the lightreflective material 612 is placed a dome 654 of a transparent materialon which the luminescent layer 652 is arranged. The radius r is thedistance between the LED 656 and the luminescent layer 652. Thedefinition of the distance h is in this case replaced by the radius r.

Light emitting module 660, depicted in FIG. 6(vi), does not comprise adome of transparent material but a box shaped transparent material 663.Further, the base 664 is made of a light reflective material and as suchno additional layer of light reflecting material is provided on thesurface of the base 664 which is facing towards the luminescent layer662. Other shapes and combinations may be envisioned as well.

The schematically shown light emitting modules 500, 520, 530, 540, 550,560, 600, 620, 630, 640, 650, 660 may be (circularly) symmetric but mayalso be asymmetric out of the plane of the depicted cross-section. Forexample, the module may be elongated in the depth direction to the planeof the paper such as to form an elongated, tube, rod, or cylinder likeshape. Multiple emitters may form an emitter array in the depthdirection. Such a shape may for instance be used in an LED streetlamp orLED retrofit TL lamp. LED emitter arrays of tens up to hundreds of LEDsmay in principle be used. Different amounts of emitters may be presentto match the light output required in the associated application.

In FIG. 7a a light emitting module 700 is shown which is manufactured ona flexible base foil 712. Solid state light emitters 706 which areprovided on a small substrate 708, which is equipped with electrodeconnection pads (not shown), are provided on the flexible base foil 712,and the area in between the substrates 708 is filled with a lightreflective material 710. The light emitters 706 are optically coupled toa layer of a flexible transparent material 704. On top of the flexiblelight transmitting material 704 a luminescent layer 702 is providedcomprising at least one luminescent material. Not the whole surface ofthe flexible light transmitting material 704 needs to be covered by theluminescent layer 702, a part of the surface may for example be blockedwith a top reflector. As seen in FIG. 7a , the light emitting module 700comprises a plurality of solid state light emitters 706. In anembodiment a relatively large two-dimensional array of solid stateemitters is provided to obtain a relatively large light exit window. Inconformity with previous embodiments, the distance between the solidstate light emitter 706 and the luminescent layer 702 should be in arange that depends on the longest linear size of the top surface of thesolid state light emitters 706, and the average reflectivity of thelight reflective surface of the base 712, formed by the combination ofthe substrates 708 and the light reflective material 710, should besubstantially larger than the reflectivity of the solid state lightemitter 706. Further, the solid state light emitters should only cover arelatively small part of the light reflective surface formed by thelight reflective material 710 and the substrates 708. It is to be notedthat the reflection coefficient Rbase of the light reflective surface isdefined as the average reflectivity of the whole light reflectivesurface. Thus, the reflection coefficient Rbase is a weighted averagebetween the reflection coefficient of the substrates and the reflectioncoefficient of the light reflective material, wherein preferably theweights are formed by the part of the total area that is covered by thespecific material.

In FIG. 7b another embodiment of a flexible light emitting module 750 ispresented. Light emitting module 750 is similar to light emitting module700, however, the base only exists of a light reflective foil 754 whichis applied to a side of a transparent material 704. On another side ofthe flexible transparent material 704, which is opposite to the side towhich the light reflective foil 754 is applied, a luminescent layer 702is arranged. Within the transparent material wires, bars or rods 752 areprovided which support substrates 708 on which solid state lightemitters 706 are provided. The wires, bars or rods 752 provideelectrical power to the solid state light emitters 706. The distancefrom a top surface of the solid state light emitters to the luminescentlayer 702 is indicated with h. The distance h is preferably larger thanor equal to 0.3 times the largest linear size d_(SSL) of the topsurfaces of the solid state light emitters 706 and smaller than or equalto 5 times the largest linear size d_(SSL) of the top surfaces of thesolid state light emitters 706 in case the solid state light emitterarea ratio ρ_(SSL) is smaller than 0.1. For a value of the solid statelight emitter area ratio ρ_(SSL) that is in a range with a minimum valuethat is larger than or equal to 0.1 and a maximum value that is smallerthan or equal to 0.25, the distance h is preferably larger than or equalto 0.15 times the largest linear size d_(SSL) of the top surfaces of thesolid state light emitters 706 and smaller than or equal to 3 times thelargest linear size d_(SSL) of the top surfaces of the solid state lightemitters 706. For a value of the solid state light emitter area ratioρ_(SSL) larger than 0.25 the distance h is preferably larger than orequal to 0.1 times the largest linear size d_(SSL) of the top surfacesof the solid state light emitters 706 and smaller than or equal to 2times the largest linear size d_(SSL) of the top surfaces of the solidstate light emitters 706. It is to be noted that this criterion alsoapplies to the light emitting module 700. Further, in conformity withpreviously discussed embodiments, the base reflection coefficient Rbaseof the light reflective foil 754 is larger than the solid state lightemitter reflection coefficient R_SSL of the solid state light emitters706, preferably the base reflection coefficient Rbase of the lightreflective foil 754 relates to the solid state light emitter reflectioncoefficient R_SSL of the solid state light emitters 706 according to:Rbase>R_SSL+c(1−R_SSL) wherein also in this case the value of the factorc depends on the solid state light emitter area ratio ρ_(SSL), which inthis case only includes the reflective area of the base as described insome of the previous embodiments.

FIGS. 8a-8c show schematic cross-sections of embodiments of lightemitting modules according to the invention. FIG. 8a shows a schematiccross-section of a light emitting module 2000 comprising LED dies 2030on a substrate carrier 2020, for example comprising alumina oraluminiumnitride. The substrate carrier 2020 is electrically connectedto contact pads of a printed circuit board 2010 via electrical contacts2015, for example solder contacts. The printed circuit board 2010 may bea metal core printed circuit board comprising an aluminium base coveredby a dielectric insulating layer (not shown). On the dielectric layerelectrically conducting electrodes and the contact pads are provided,and the electrodes are protected by a solder mask protective layer (notshown). The LED dies 2030 are covered with a transparent protectivelayer 2035, for example a transparent silicone layer. In between the LEDpackages or devices, which comprise the LED dies 2030, the substratecarrier 2020 and the transparent protective layer 2035, a reflectivelayer 2040 is provided, for example a white TiO2 pigmented silicone. Acavity is defined by the printed circuit board 2010, walls 2050 and aluminescent layer 2060. The walls 2050 for example comprise TiO2dispensed in silicone and the luminescent layer 2060 for examplecomprises a phosphor material. On the transparent protective layer 2035and the reflective layer 2040 an optical bond layer 2045 is provided,comprising for example silicone, which provides for optical bondingbetween the transparent protective layer 2035 and a filling layer 2055,wherein the filling layer 2055 comprises, for example, glass andsubstantially fills the cavity in between the optical bond layer 2045,the luminescent layer 2060 and the walls 2050.

FIG. 8b shows a schematic cross-section of a light emitting module 2100comprising LED dies 2130 on a substrate carrier 2120, for examplecomprising alumina or aluminiumnitride. The substrate carrier 2120 iselectrically connected to contact pads of a printed circuit board 2110via electrical contacts 2115, for example solder contacts. The printedcircuit board 2110 may be a metal core printed circuit board asdescribed in relation to the light emitting module 2000 which isdepicted in FIG. 8a . The LED dies 2130 are covered with a transparentprotective layer 2135, for example a transparent silicone layer. On thetransparent protective layer 2135 of each of the LED dies 2135 anoptical bond layer 2145 is provided, comprising for example silicone. Inbetween the LED packages or devices with the optical bond layer 2145,wherein the LED packages or devices comprise the LED dies 2130, thesubstrate carrier 2120 and the transparent protective layer 2135, areflective layer 2140 is provided, for example a white TiO2 pigmentedsilicone. A cavity is defined by the printed circuit board 2110, walls2150 and a luminescent layer 2160. The walls 2150 for example compriseTiO2 dispensed in silicone and the luminescent layer 2160 for examplecomprises a phosphor material. A filling layer 2155, which comprises,for example, glass, substantially fills the cavity in between theoptical bond layer 2145, the reflective layer 2140, the luminescentlayer 2160 and the walls 2150. The optical bond layer 2145 provides foroptical bonding between the transparent protective layer 2135 and thefilling layer 2155. The reflective layer 2140 is provided in between theLED packages or devices via, for example, underfilling or overmoldingafter bonding of the LED packages or devices to the filling layer 2155via the optical bond layer 2145.

FIG. 8c shows a schematic cross-section of a light emitting module 2300comprising LED dies 2330 on a substrate carrier 2320, for examplecomprising alumina or aluminiumnitride. The substrate carrier 2320 iselectrically connected to contact pads of a printed circuit board 2310via electrical contacts 2315, for example solder contacts. The printedcircuit board 2310 may be a metal core printed circuit board asdescribed in relation to the light emitting module 2000 which isdepicted in FIG. 8a . In between the LED packages or devices, whichcomprise the LED dies 2330 and the substrate carriers 2320, a reflectivelayer 2340 is provided, for example a white TiO2 pigmented silicone. Acavity is defined by the printed circuit board 2310, walls 2350 and aluminescent layer 2360. The walls 2350 for example comprise TiO2dispensed in silicone and the luminescent layer 2360 for examplecomprises a phosphor material. On the reflective layer 2340 and the LEDdevices or packages an optical bond layer 2345 is provided, comprisingfor example silicone, which provides for optical bonding between the LEDpackages or devices and a filling layer 2355, wherein the filling layer2355 comprises, for example, glass and substantially fills the cavity inbetween the optical bond layer 2345, the luminescent layer 2360 and thewalls 2350. This light emitting module 2300 differs from the lightemitting module 2000, which is depicted in FIG. 8a , in that the LEDdies 2330 are not covered with a transparent protective layer but arecovered with the optical bond layer 2345.

Several embodiments were manufactured according to the invention. In afirst experiment a Philips Fortimo SLM light emitting module was used asa reference comprising 16 LEDs with phosphor directly on top of the dieswith a luminous flux of 1800 lumen. Light emitting modules according toan embodiment of the invention contained 16 blue light emitting LEDs ina highly reflective mixing chamber with a Lumiramic™ phosphor layer at adistance of 2.1 mm from the LEDs and with a cavity diameter of 22 mm. At640 mA the Wall Plug Efficiency (WPE) was improved with a factor rangingbetween 30% and 50%. The Wall Plug Efficiency is the energy conversionefficiency with which electrical power is converted into optical power(in Watt) and is also defined as the ratio of the radiant flux (i.e.radiant energy per unit time, also called radiant power) to the inputelectrical power. FIG. 9 shows the results of measurements that wereperformed at varying current levels on one of the light emitting modulesaccording to an embodiment of the invention having 16 LEDs. Thehorizontal x-axis represents the current level and the vertical y-axisrepresents the gain or improvement of the radiant flux of one of thelight emitting modules according to an embodiment of the inventionhaving 16 LEDs with respect to the radiant flux of the reference lightemitting module with 16 LEDs having phosphor directly on the LEDs. FIG.9 shows that the improvement of the radiant flux with respect to thereference light emitting module increases when the current increases,which can be attributed to an improved photo-thermal performance of thephosphor layer with respect to the reference.

In another experiment light emitting modules were fabricated accordingto an embodiment of the invention comprising 9 LEDs each with a topsurface area of 1 mm² and comprising 4 LEDs each with a top surface areaof 2 mm² each with a Lumiramic™ phosphor layer at a distance of 2.1 mmfrom the LEDs. Measurements of the radiant flux showed improvements ofthe radiant flux with respect to the reference light emitting modulewith 16 LEDs with phosphor directly on top ranging between 20% and 40%.

FIGS. 10a-c show schematic cross-sections of another comparativeexperiment. FIG. 10a shows a schematic cross-section of a firstreference light emitting module 850 comprising four LEDs 852 (one LEDnot shown) with a luminescent layer 853 directly on top placed on a basesubstrate 851. Each LED is covered with a dome-shaped optical element854. FIG. 10b shows a schematic cross-section of a second referencelight emitting module 860 which differs from the first reference lightemitting module because of a reflective layer 855 which is applied onthe base substrate 851 in between the LEDs. FIG. 10c shows a schematiccross-section of a light emitting module 870 according to an embodimentof the invention comprising four LEDs 872 (one LED not shown) on a basesubstrate 871 that is covered with a reflective layer 875. The LEDs areplaced in a cavity 874 that is covered with a luminescent layer 873 at adistance of 2.1 mm from the top surface of the LEDs 872. Measurements ofthe radiant flux show an improvement of the radiant flux of the secondreference light emitting module 860 with respect to the radiant flux ofthe first reference light emitting module 850 of approximately 4%(measured at 700 mA), which is mainly due to the additional reflectivelayer 855 in the second reference light emitting module 860. Themeasured improvement of the radiant flux of the light emitting module870 according to an embodiment of the invention with respect to theradiant flux of the first reference light emitting module 850 isapproximately 25% (measured at 700 mA).

FIGS. 11, 12, 13 and 14 show graphs with the results of simulations of alight emitting module according to the invention. With a light-raytracing software package an optical model was built of the lightemitting modules according to the invention. The model comprises sevenblue light emitting LEDs with dies that have a top surface of 1×1 mm²each. Thus, the largest linear size d_(SSL) of the top surfaces of theseLEDs is about 1.4 mm. The LED dies have a diffuse reflectance with aweighted average over the first and second spectral range of about 70%,which corresponds to a typical surface roughened GaN type of LED die.The cavity has a circular shape with a varying diameter. The LEDs areuniformly distributed on a highly reflective substrate and aresurrounded with highly reflective walls forming the cavity. The lightexit window of the cavity is covered by a luminescent layer comprising aceramic phosphor and an additional coating layer with particles ofanother phosphor in silicon. The light emitted by the light emittingmodule through the light exit window has a warm white color point.

The optical simulations show that the walls and/or base either beingdiffuse reflective or specular reflective, or combinations thereof, hasa minor influence, in the order of a few percent, on the performance ofthe light emitting module. This influence depends, amongst others, onthe area ratio and the geometry of the cavity.

FIG. 11 shows the influence of the factor c on the optical efficiencyfor several values of the solid state emitter area ratio. In FIG. 11 thevertical y-axis represents the optimum value of the efficiency of theoptical performance expressed by the ratio of the flux of whiteradiation exiting the mixing cavity Wwhite (units: Watt) and the totalblue flux emitted by the solid state light emitters in first spectralrange, usually the blue spectral range, Wblue (units: Watt). The optimumvalue of the optical efficiency is determined by varying the distance hbetween the solid state light emitter top surface and the luminescentlayer. The horizontal x-axis represents the factor c from the formulaReff>R_SSL+c*(1−R_SSL). Curve 801 represents a range of relatively lowvalues of the solid state light emitter area ratio ρ_(SSL), in this casevarying between 0.01 and 0.02, curve 802 represents an intermediatevalue range of the solid state light emitter area ratio ρ_(SSL), in thiscase varying between 0.19 and 0.28, and curve 803 represents a rangewith relatively high values of the solid state light emitter area ratioρ_(SSL), in this case varying between 0.39 and 0.68. A reference lightemitting module with a luminescent layer directly on top of the LEDsshows an optical efficiency of about 0.5, thus an efficiency improvementover the reference light emitting module is in this case achieved for avalue of the optical efficiency that is larger than 0.5. FIG. 11 showsthat the factor c should be larger than about 0.2 to have an opticalefficiency larger than 0.5 in the relatively low value range of thesolid state light emitter area ratio ρ_(SSL), larger than about 0.3 tohave an optical efficiency larger than 0.5 in the intermediate valuerange of the solid state light emitter area ratio ρ_(SSL), larger thanabout 0.4 to have an optical efficiency larger than 0.5 in therelatively high value range of the solid state light emitter area ratioρ_(SSL). Even better values of the optical efficiencies can be reachedfor larger values of the factor c in the respective ranges of the solidstate light emitter area ratio ρ_(SSL).

FIG. 12 shows the dependence of the optimum distance h, indicated asHopt in the graph, on the reflection coefficient of the cavity wallsRwall. The optimum distance Hopt is the distance h between the topsurface of the solid state light emitter and the luminescent layer wherethe optical efficiency of the light emitting module is optimal, e.g. hasa local maximum. In FIG. 12 the vertical y-axis represents the quotientof the optimum distance Hopt and the largest linear size of the LEDd_(SSL) and the horizontal x-axis represents the reflection coefficientof the cavity walls Rwall in %. In this case the LED area ratio ρ_(SSL)with respect to the base and walls varies for each of the curves,because each of the curves 811, 812, 813 represent a variable distance hbetween the LED top surface and the luminescent layer, and hence avariable height of the walls and thus the LED area ratio ρ_(SSL) withrespect to the total reflective area of the walls and the base varies.For curve 811 the total LED area ratio ρ_(SSL) varies between 0.01 and0.02, for curve 812 the total LED area ratio ρ_(SSL) varies between 0.16and 0.22 and for curve 813 the total LED area ratio ρ_(SSL) variesbetween 0.28 and 0.41. The reflection coefficient of the base Rbase inthis case is in a range between 85% and 95%. The optimum distance Hoptbetween the luminescent layer and the LEDs is determined by a balance oflight absorption losses in LEDs and cavity walls. At relatively lowvalues of the distance h between the LED top surface and the luminescentlayer the light emitted by the LEDs will interact dominantly with theLED, LED substrate and the surface of base reflector in the LED. Atrelatively large values of the distance h between the LED top surfaceand the luminescent layer the area of the walls will become dominant andabsorption losses will be dominated by the walls. The optimum distanceHopt between the luminescent layer and the LEDs depends mostly on thereflection coefficient of the surfaces of the walls Rwall and the LEDarea ratio parameter ρ_(SSL). On average for a relatively low LED arearatio ρ_(SSL) and typical values of the wall reflection coefficientRwall, for example in the range of 80% to 90%, the optimum distance Hoptis of the order of half of the largest linear size of the LED d_(SSL).Increasing the value of the wall reflection coefficient Rwall, forexample to above 95%, results in an increase of the optimum distanceHopt between the LEDs and the luminescent layer. Increasing the LED arearatio ρ_(SSL) results in a decrease of the optimum distance Hopt. It wasfound that a relatively efficient lighting module is provided for ifRwall<95% and 0.3*d_(SSL)≦h≦0.75*d_(SSL) for 0<ρ_(SSL)<0.1,0.15*d_(SSL)≦h≦0.3*d_(SSL) for 0.1≦ρ_(SSL)<0.25 and0.1*d_(SSL)≦h≦0.2*d_(SSL) for ρ_(SSL)>0.25. Furthermore it was foundthat in the case that Rwall≧95% a relatively efficient lighting moduleis provided for when the lighting module fulfills the followingcriteria: 0.75*d_(SSL)≦h≦2*d_(SSL) for 0<ρ_(SSL)<0.1, 0.3*d_(SSL)≦h≦0.7*d_(SSL) for 0.1≦ρ_(SSL)≦0.25 and 0.2*d_(SSL)≦h≦0.5*d_(SSL) forρ_(SSL)>0.25. The results of FIG. 12 consider only cavities with wallsorthogonal to the base and a uniform LED placement. For tilted wallsand/or non uniform LED placement the optimum distance between the LEDsand the luminescent layer may increase.

FIG. 13 shows the influence of the total solid state light emitter arearatio ρ_(SSL) on the optical efficiency for several combinations of baseand wall reflection coefficients. In FIG. 13 the vertical y-axisrepresents the optimum value of the efficiency of the opticalperformance expressed by the ratio of the flux of white radiationexiting the mixing cavity Wwhite (units: Watt) and the total blue fluxemitted by the solid state light emitters in first spectral range,usually the blue spectral range, Wblue (units: Watt). The optimum valueof the optical efficiency is determined by varying the distance hbetween the solid state light emitter top surface and the luminescentlayer. The horizontal x-axis represents the solid state light emitterarea ratio ρ_(SSL) with respect to the base and wall area. In total sixcurves 821, 822, 823, 824, 825, 826 are shown for two different valuesof the base reflection coefficient Rbase and three different values ofthe reflection coefficient of the cavity walls Rwall. Curve 821represents Rbase=80% and Rwall=90%, curve 822 represents Rbase=80% andRwall=98%, curve 823 represents Rbase=90% and Rwall=90%, curve 824represents Rbase=90% and Rwall=98%, curve 825 represents Rbase=98% andRwall=90% and curve 826 represents Rbase=98% and Rwall=98%. FIG. 13shows that there is an inverse relation between the optimum value of theoptical efficiency of the light mixing cavity and the solid state lightemitter area ratio ρ_(SSL). FIG. 13 further shows that three ranges ofthe solid state light emitter area ratio ρ_(SSL) values can bedistinguished: a relatively low, an intermediate and a relatively highrange of the values of the solid state light emitter area ratio ρ_(SSL).At relatively low values of ρ_(SSL), for example ρ_(SSL)<0.1, theinfluence of the value of the reflection coefficient of the wall Rwallon the value of the optical efficiency is almost negligible compared tothe influence of the value of the reflection coefficient of the baseRbase, i.e. changing the value of the base reflection coefficient Rbasehas an impact on the optical efficiency of the light emitting module anda change of the value of the wall reflection coefficient Rwall impactsthe optical efficiency in a negligible way in this relatively low valuerange of the solid state light emitter area ratio ρ_(SSL). At relativelyhigh values of PSSL, for example if ρ_(SSL)>0.25, the influence of thevalue of the reflection coefficient of the walls Rwall on the value ofthe optical efficiency is comparable to the influence of the reflectioncoefficient of the base Rbase, i.e. changing the value of the basereflection coefficient Rbase has a comparable impact on the opticalefficiency of the light emitting module as a change of the value of thewall reflection coefficient Rwall in this high value range of the solidstate light emitter area ratio ρ_(SSL). At intermediate values ofρ_(SSL), for example 0.1≦ρ_(SSL)≦0.25, the influence of the reflectioncoefficient of the base Rbase on the value of the optical efficiency islarger than the, in this range non-negligible, influence of the value ofthe reflection coefficient of the walls Rwall, i.e. changing the valueof the base reflection coefficient Rbase has an impact on the opticalefficiency of the light emitting module and a change of the value of thewall reflection coefficient Rwall also impacts the optical efficiencybut to a lesser extent in this intermediate value range of the solidstate light emitter area ratio ρ_(SSL).

FIG. 14 shows the dependence of a maximum possible solid state lightemitter area ratio at which a gain in optical efficiency is achieved asa function of the effective reflectivity coefficient Reff of the baseand the walls according to an aspect of the invention. The verticaly-axis in FIG. 14 represents the maximum possible solid state lightemitter area ratio, indicated as ρ_(SSL,MAX), at which an improvedoptical efficiency is achieved with respect to a lighting emittingmodule with the luminescent layer placed directly on top of the solidstate light emitter. The horizontal x-axis represents the effectivereflectivity coefficient of the cavity base and walls surfaces Reff. Theset of data points 831 represents a distance h between the solid statelight emitter surface and the luminescent layer of 0.35 times thelargest linear size d_(SSL) of the solid state light emitter, the set ofdata points 832 represents a distance h of 1.04 times the largest linearsize d_(SSL) of the solid state light emitter and the set of data points833 represents a distance h of 1.73 times the largest linear sized_(SSL) of the solid state light emitter. The results allow to predictthe maximum possible solid state light emitter area ratio PSSL, MAX at acertain distance h which still allows for a relatively large lightrecycling efficiency and relatively good performance compared to thesame number of solid state light emitters with the luminescent layerplaced directly on the solid state light emitters. From FIG. 14 it canbe concluded that a larger value of the effective reflection coefficientReff allows for a larger value of the solid state light emitter arearatio PSSL, MAX (depending on the distance h between the solid statelight emitter top surface and the luminescent layer) while stillachieving an improved optical efficiency with respect to the referencesituation in which the luminescent layer is placed directly on the solidstate light emitters. Increasing the distance h between the solid statelight emitter and the luminescent layer decreases the maximum allowedsolid state light emitter area ratio ρ_(SSL,MAX) at similar values ofthe effective reflection coefficient Reff which still provides for animproved optical efficiency with respect to the reference situation inwhich the luminescent layer is placed directly on the solid state lightemitters.

FIG. 15 shows a comparison of optical modeling results for a lightemitting module according to the invention with walls orthogonal to thebase and cavities with tilted walls. The results were obtained fromoptical simulation modeling with four LEDs each having a die area of 2mm². The diameter of the luminescent layer is 6.5 mm and the LED arearatio ρ_(SSL) with respect to the base area only is 0.241 and 0.298 forthe orthogonal and tilted walls respectively. Also in this case the LEDarea ratio ρ_(SSL) with respect to the base and the walls varies as afunction of the distance h between the LED and the luminescent layer.The angle between the reflective surface of the tilted walls and thereflective surface of the base is in this case in a range of 5 to 33degrees. In FIG. 15 the vertical y-axis represents the efficiency of theoptical performance expressed as the ratio of the flux of whiteradiation exiting the mixing cavity Wwhite (units: Watt) and the totalblue flux emitted by the solid state light emitters in blue spectralrange Wblue (units: Watt) and the horizontal x-axis represents thedistance h between the LED top surface and the luminescent layer inmillimeters. Curve 841 represents the light emitting module with theorthogonal walls and curve 840 represents the light emitting module withthe inclined or tilted walls. It is clear that a relatively largeoptical efficiency can be achieved by the tilting of the walls for thisembodiment in which the LED area ratio ρ_(SSL) is in the intermediatevalue range. The optimum value of the optical efficiency in this case isachieved at a distance h between the LED top surface and the luminescentlayer of approximately 1.1 mm and 0.75 mm for tilted and orthogonalwalls respectively, at which distance the LED area ratio ρ_(SSL) withrespect to the base and the walls is 0.18 for the light emitting modulewith the straight walls and 0.21 for the light emitting module with theinclined or tilted walls. For light emitting modules with anintermediate LED area ratio ρ_(SSL) a significant amount of lightreflected from the walls can impinge on the poor reflecting LED area.Tilting of the walls improves the situation by a more efficientre-direction of light toward the exit window comprising the luminescentlayer which results in relatively large Wwhite/Wblue values and thus animproved optical efficiency. This effect becomes more pronounced forrelatively large values of the LED area ratio ρ_(SSL). For relativelysmall values of the LED area ratio ρ_(SSL) the walls are more distantfrom the LEDs and tilting of the walls will have a relatively smalleffect on the optical efficiency.

From manufacturing point of view, LED dies can be positioned on a highlyreflective PCB board, without filling the space in between LED packageswith white reflecting material. In this case the reflective surface ofthe base can be positioned on a significantly lower level than thesurface of LED dies. The influence of the distance h between the topsurface of the LED and the luminescent layer and the distance betweenthe surface of the reflective base and the luminescent layer, indicatedas h2, on the optimum position of the luminescent layer was investigatedby means of optical ray-trace modeling of a light emitting moduleaccording to the invention with one LED. FIG. 16a shows a cross-sectionof a first light emitting module 900 having a base 906, a solid statelight emitter 908, for example a LED, and a reflective base surface 901that is further removed from a luminescent layer 902 than a top surface903 of the LED 908, i.e. h2>h. FIG. 16b shows a cross-section of asecond light emitting module 910, for example a LED, in which thereflective base surface 901 is closer to the luminescent layer 902 thanthe top surface 903 of the LED, i.e. h2<h. In the latter case there is aconical opening or recess in the center of reflective base, with anangle of for example 45 degrees.

FIG. 16c shows the results of simulations in which the vertical y-axisrepresents the efficiency of the optical performance expressed as theratio of the flux of white radiation exiting the mixing cavity Wwhite(units: Watt) and the total blue flux emitted by the solid state lightemitter 908 in the first, blue, spectral range Wblue (units: Watt) andthe horizontal x-axis represents the distance h between the luminescentlayer 902 and the top surface 903 of the solid state light emitter 908.FIG. 16c shows seven curves 951, 952, 953, 954, 955, 956, 957 eachrepresenting a different value for the difference between the distanceh2 between the reflective base surface 901 and the luminescent layer 902and the distance h between the LED top surface 903 and the luminescentlayer 902. Curves 951, 952 and 953 represent variations of the firstlight emitting module 900 in which the reflective base surface 901 isfurther away from the luminescent layer 902 than the LED top surface903, i.e. h2>h: curve 951 represents h2=h+1.5 mm, curve 952 representsh2=h+1.0 mm and curve 953 represents h2=h+0.5 mm. Curve 954 representsthe situation in which the distance h2 between the reflective basesurface 901 and the luminescent layer 902 is equal to the distance hbetween the LED top surface 903 and the luminescent layer 902, i.e.h2=h. Curves 955, 956 and 957 represent variations of the second lightemitting module 910 in which the reflective base surface 901 is closerto the luminescent layer 902 than the LED top surface 903, i.e. h2<h:curve 955 represents h2=h−0.5 mm, curve 956 represents h2=h−1.0 mm andcurve 957 represents h2=h⁻¹.5 mm. From the curves in FIG. 16c it can beconcluded that for the LED device 910 in which the reflective basesurface 901 is closer to the luminescent layer 902 than the top surface903 of the LED, i.e. h2<h, the optimum value of the distance h betweenthe LED top surface 903 and the luminescent layer 902, which is thevalue of the distance h for which the optical efficiency has an optimum,e.g. a local maximum, is almost independent of the distance h2 betweenthe reflective base surface 901 and the luminescent layer 902. Thus, thecriteria for the distance h between the top surface of the solid statelight emitter and the luminescent layer, as defined above, can also beapplied for this first light emitting module 900. When the reflectivebase surface 901 is closer to the luminescent layer 902 than the topsurface 903 of the LED, for example in the case that the LED is placedin a recess in the reflective base, i.e. h>h2, the distance h at whichthe efficiency has an optimum is larger with respect to the situation inwhich the reflective base surface 901 and the LED surface 903 have anequal distance to the luminescent layer 902. For the second lightemitting module 910 where the reflective base surface 901 is closer tothe luminescent layer 902 than the top surface 903 of the LED, i.e.,h2<h, the criteria for the distance change to: 0.4*d+Δh/2<h<5*d+Δh/2 forρ_(SSL)<0.1, 0.15*d+Δh/2<h<3*d+Δh/2 for 0.1≦ρ_(SSL)≦0.25, and0.1*d+Δh/2<h<2*d+Δh/2 for ρ_(SSL)>0.25, in which Δh is the absolutevalue of the distance between the surface of the reflective base 901 andthe top surface 903 of the LED, i.e. Δh=|h2−h|.

The relative positioning or placement of the plurality of solid statelight emitters on the base is another design parameter. The placement ofthe solid state light emitters in the cavity may influence thedistribution and uniformity of the optical flux in the exit window ofthe cavity comprising the luminescent layer. It is desirable to avoidoptical hot spots which may result in thermal hot spots. This isespecially important for the center of the cavity where the heat load inthe luminescent layer is more difficult to transport to a PCB board andto a heat sink for example because of the relatively long distanceand/or due to the relatively low thermal conductivity of the opticalmaterial filling the cavity, with respect to the, in some embodiments,relatively high thermal conductivity of the cavity walls.

The influence of different LED distributions inside the cavity onefficiency and on optimum distance between the LEDs and the luminescentlayer is investigated with optical ray-trace modeling of a lightemitting module according to the invention. FIG. 17a shows a schematictop view of a light emitting module 980 with a wall 981 and a basesurface 982 in which one LED 984 is positioned in the center of the base982 and six other LEDs 983 are positioned on an imaginary circle with aradius of placement r_(pl), equidistant from the center and equidistantfrom each other. The light emitting module 980 in this case comprisesseven LEDs each having an area of 1×1 mm². The calculations areperformed for three different values base radius r_(base), respectively7.46 mm, 3.05 mm and 2.36 mm. The distance between the LED top surfaceand the luminescent layer has been varied, resulting in different heightvalues of the walls, and thus in different areas of the wall. Thereforethe value of the solid state emitter area ratio ρ_(SSL) with respect tothe base and walls ranges between 0.02 and 0.04 for the case wherer_(base) is 7.46 mm, between 0.09 and 0.22 for the case where r_(base)is 3.05 mm and between 0.13 and 0.39 for the case where r_(base) is 2.36mm. FIGS. 17b, 17c and 17d show the results of the optical ray tracingsimulations in which the vertical y-axis represents the efficiency ofthe optical performance expressed as the ratio of the flux of whiteradiation exiting the mixing cavity Wwhite (units: Watt) and the totalblue flux emitted by the solid state light emitter in blue spectralrange Wblue (units: Watt) and the horizontal x-axis represents thedistance h in millimeters between the luminescent layer and the LED topsurface. The different curves in FIGS. 17b, 17c and 17d representdifferent values of the placement radius r_(pl). In FIG. 17b shows theresults at a base radius r_(base) of 7.46 mm and curve 1101 representsr_(pl)=1.2 mm, curve 1102 represents r_(pl)=1.5 mm, curve 1103represents r_(pl)=2.5 mm, curve 1104 represents r_(pl)=3.5 mm, curve1105 represents r_(pl)=4.5 mm, curve 1106 represents r_(pl)=5.5 mm andcurve 1107 represents r_(pl)=6.5 mm. In FIG. 17c shows the results at abase radius r_(base) of 3.05 mm and curve 1111 represents r_(pl)=1.2 mm,curve 1112 represents r_(pl)=1.4 mm, curve 1113 represents r_(pl)=1.8 mmand curve 1114 represents r_(pl)=2.2 mm. In FIG. 17 d shows the resultsat a base radius r_(base) of 2.36 mm and curve 1121 representsr_(pl)=1.2 mm, curve 1122 represents r_(pl)=1.4 mm and curve 1123represents r_(pl)=1.6 mm.

A comparison of the curves of FIG. 17b with those of FIGS. 17c and 17dshows that the influence of different LED positioning on the opticalefficiency and the optimum LED top surface to luminescent layerdistance, where the efficiency has an optimum, is more pronounced forthe cavity with the relatively low LED area ratio ρ_(SSL), the resultsof which are shown in FIG. 17b . FIG. 17b further shows two extremecases of LED placement, in which the outer LEDs are placed relativelyclose to the center, corresponding to the lowest value of the radius ofplacement r_(pl) and curve 1101, or relatively close to the walls,corresponding to the largest value of the radius of placement r_(pl) andcurve 1107. Both extremes cases result in a relatively low value of theoptical efficiency.

When the LEDs are placed relatively close together, such that the spacein between the LEDs is comparable to the size of the LEDs, then thereflectivity of the base surface around each of the LEDs reducessignificantly and the situation may be approximated with a model of onelarge LED die (a multi-die LED). In this multi die LED situation theoptimum distance between the LED top surfaces and the luminescent layeris increased to achieve an efficient recycling of light, which isclearly visible for the light emitting module with the relatively lowLED area ratio ρ_(SSL) (see FIG. 17b ). This effect is less pronouncedfor the light emitting modules with the medium to high LED area ratioρ_(SSL) (FIGS. 17c and 17d ). For these latter light emitting modulesthere is less influence of the LED placement on the optical efficiency,viz. placing the LEDs closer to the center or closer to the cavity wallshas less influence on the optical efficiency, than the light emittingmodules with a relatively low value of the LED area ratio ρ_(SSL).

For optical efficiency reasons it is preferred to place the solid statelight emitters equidistant from each other and equidistant from thewalls. Non-uniform solid state light emitter placement results in hotspots and also increases the light absorption losses in the solid statelight emitters. A relatively high value of the solid state light emitterarea ratio ρ_(SSL) reduces the sensitivity of the optical efficiencyWwhite/Wblue on the placement of the solid state light emitters, alsobecause in this case there is less physical space on the light emittingmodule for changing the location of the LED. The value of the optimumdistance between the solid state light emitter top surface and theluminescent layer, which corresponds to the distance resulting in thehighest optical efficiency, is generally lower for relatively largevalues of the solid state light emitter area ratio ρ_(SSL).

In order to achieve high efficiency values of the cavity it is preferredthat all surfaces inside the cavity are highly reflective over the wholespectral range of the device. For this purpose not only wall surfacesbut also spaces in between LED packages and LED substrates themselvesare additionally coated with, for example, white reflecting coating, forexample TiO₂ filled silicone. For practical reasons the step of applyinga reflective coating on the LED packages is difficult. Therefore thereflectivity coefficients of surfaces next to the LED other than thebase surface are in practice relatively low.

FIG. 18 shows the results of optical ray tracing simulations in whichthe vertical y-axis represents the efficiency of the optical performanceexpressed as the ratio of the flux of white radiation exiting the mixingcavity Wwhite (units: Watt) and the total blue flux emitted by the solidstate light emitter in blue spectral range Wblue (units: Watt) and thehorizontal x-axis represents the distance h in millimeters between theluminescent layer and the top surface of the LED. The simulationsincluded four LEDs each having a die area of 2 mm² and the diameter ofthe luminescent layer is 6.5 mm. Curve 1152 represents an uncoated LEDpackage and curve 1151 represents a LED package coated with a reflectivelayer. FIG. 18 shows that uncoated LED packages have a somewhat loweroptical efficiency with respect to the LED packages with a reflectivecoating, but no significant changes in the optimum LED surface toluminescent layer distance h were observed. These simulation resultswere verified by an experiment with uncoated and coated LED packageswhich showed an increase of approximately 7% in optical efficiency forthe LED package with the reflective coating with respect to the uncoatedLED package.

FIG. 19a shows an embodiment of a lamp 1000 according to the secondaspect of the invention. The lamp 1000 comprises a retrofit light bulb1002 which is connected to a lamp base 1006 which includes a heat sink,a power driver and electrical connections. On the lamp base 1006 isprovided a light emitting module 1004 according to the first aspect ofthe invention. It is to be noted that embodiments of the lamp are notlimited to lamps that have the size of a traditional light bulb. Othershapes, likes tube, are possible as well. Alternative lamp types, such aspot lamps or downlighter may be used as well. The lamps may comprise aplurality of light emitting modules as well.

FIG. 19b shows another embodiment of a lamp 1020. Lamp 1020 is a spotlamp which comprises a reflector 1022 for collimating the light which isemitted by a light emitting module 1004. The light emitting module 1004is thermally coupled to a heat sink 1024 for conducting the heat awayfrom the light emitting module 1004 and providing the heat to theambient of the lamp 1020. The heat sink 1024 may be passively oractively cooled.

FIG. 19c shows an embodiment of a luminaire 1050 according to the thirdaspect of the invention. The luminaire 1050 comprises a light emittingmodule 1052 according to the first aspect of the invention. In otherembodiments, the luminaire 1050 comprises a lamp according to the secondaspect of the invention.

The lamp according to the second aspect of the invention and theluminaire according to the third aspect of the invention, have similarembodiments with similar effects as the light emitting module of thefirst aspect of the invention as described with reference to FIGS. 1-18.

FIG. 20 presents another embodiment of the light emitting moduleaccording to the invention. Light emitting module 1300 comprises a base518, a plurality of light emitting diodes 514 provided on substrates524, a luminescent layer 506, reflective walls 522, a transparentmaterial 502, and light reflective particles filled layer 512, similarto light emitting module 520. However, a layer of air 1301 and apolarizing element 1302 are positioned on the luminescent layer 506 atthe side facing away from the light emitting diodes 514. Light emittingmodule 1300 generates, in use, polarized light that can be used, amongstothers, for street lighting, office lighting and retail lighting, and iscapable of reducing the amount of glare in these applications.Alternatively, it can be used in liquid crystal display (LCD)backlighting applications, reducing the cost level as no separatepolarizer is necessary any more. Light exiting the luminescent layer 506and impinging on the polarizing element 1302 with the correctpolarization is transmitted through the polarizing element, while lightwith the non-correct polarization is directed back into the luminescentlayer 506 and the cavity of the light emitting module 1300. This lightwill be randomly polarized or depolarized by scattering in theluminescent layer 506 and/or by diffusive reflection inside the cavityvia the reflective walls 522 and/or the light reflective particlesfilled layer 512, reflected again in the direction of the polarizingelement 1302, and light with the correct polarization will betransmitted through the polarizing element 1302. The non-transmittedlight is directed back again into the luminescent layer 506 and thecavity, where this process is repeated. Due the relatively high lightrecycling efficiency of the cavity, the light emitting module 1300 is arelatively efficient polarized light source. The polarizing element 1302is separated from the luminescent layer 506 by a layer of air 1301 toimprove the thermal stability of the polarizing element 1302 due to heatgenerated in the luminescent layer 506. In an alternative embodiment,the polarizing element 1302 is in direct contact with the luminescentlayer 506, for example on a ceramic layer that contains the luminescentmaterial. The polarizing element 1302 may be a reflective or ascattering polarizer. The polarizing element 1302 may be a reflectivepolarizing foil, for example commercially available Vikuity DBEF foilfrom 3M. Alternatively, polarizing element 1302 comprises narrow metallines with high reflectivity, for example commercially available fromMoxtek. By varying the width of the metal lines and/or the pitch betweenthe metal lines, the amount of polarization versus light transmissioncan be optimized.

FIG. 21 shows an embodiment of a display device 1400 according to thefourth aspect of the invention. The display device comprises a lightemitting module according to the invention as described with referenceto FIGS. 1 to 18 and 20. In use, the light emitting module may act as abacklighting unit for a LCD display device or as a light source unit toinject polarized light in a lightguide layer of the backlight system. Asthe light emitting module generates relatively efficient (polarizedlight), the cost level of the display device 1400 is reduced.

In all applicable embodiments, in the cavity a solid state light emittermay be provided which emits light in at least one sideward direction.The sideward emission is typically obtained by providing two additionallayers on top of a general purpose solid state light emitter, which area layer of a transparent material and a layer of a light reflectivematerial. A LED, which is a solid state light emitter, is oftenmanufactured on a substrate of transparent sapphire. Aftermanufacturing, in many cases, the layer of sapphire is removed. However,when the sapphire is not removed, or only partially removed, theaddition of a light reflective coating to a surface of the sapphirelayer which is substantially opposite to the LED results in themanufacturing of the sideward emitting solid state light emitter.Alternatively, a piece of glass or sapphire may be adhered to the LED.

In another embodiment one or more additional solid state lightemitter(s) may be provided on the wall of the light emitting module. Inthat case the reflective area of the wall surface should be corrected bythe area of the solid state light emitter(s) that are provided on thewall of the light emitting module.

In an embodiment the light emitting module further comprises a domeshape or lens shape optical body may be present on a side of thepartially diffusive reflective layer which faces away from the lightexit window. Alternatively, or additionally, a diffuser layer forobtaining a diffuse light emission, for obtaining a spatially, color andcolor over-angle uniform light emission, and for obtaining a color mixedlight emission is provided at a distance from the side of the partiallydiffusive reflective layer facing away from the at least one solid statelight emitter.

For all applicable embodiments the walls and base may be manufactured ofthe one and the same material and glued together. In another embodiment,the walls and base are different materials. It is to be noted that thebase, as drawn, may extend beyond the walls, for example, when one baseis shared by a plurality of neighboring light emitting modules, forexample, when the base is a heat conducting Printed Circuit Board.

The invention can typically be applied on a module level, for examplePCB board, that comprises at least one but typically multiple LEDpackages. However the invention may also be used on LED packages,comprising either one or more than one LED dies or chips. Also the LEDdies or chips may comprise the so-called Chip-on-Boards type in whichthe LED dies are directly attached to a (PCB) board without intermediateLED packages. Additionally wire-bond connections from the LED die(s) tothe board may be used.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. Use of the verb “comprise” and itsconjugations does not exclude the presence of elements or steps otherthan those stated in a claim. The article “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention may be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. Inthe device claim enumerating several means, several of these means maybe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

The invention claimed is:
 1. A light emitting module comprising: a lightexit window; a base comprising a light reflective surface facing thelight exit window, the light reflective surface having a base reflectioncoefficient (Rbase) being defined by a ratio between the amount of lightthat is reflected by the light reflective surface of the base and theamount of light that impinges on the light reflective surface of thebase; at least one solid state light emitter disposed on the basearranged for emitting light of a first color, the at least one solidstate light emitter having a top surface and having a solid state lightemitter reflection coefficient (R_SSL) being defined by a ratio betweenthe amount of light that is reflected by the top surface and the amountof light that impinges on the top surface, wherein the top surface ofthe at least one solid state light emitter faces towards the light exitwindow, a wall interposed between the base and the light exit window,the base, the wall and the light exit window enclosing a cavity, thewall comprising a reflective material forming a light reflective wallsurface facing towards the cavity, the light reflective wall surfacehaving a wall reflection coefficient (Rwall) being defined by a ratiobetween the amount of light that is reflected by the light reflectivewall surface and the amount of light that impinges on the lightreflective wall surface, wherein an effective reflective coefficient(Reff) is defined as a weighted average of the base reflectioncoefficient (Rbase) and the wall reflection coefficient (Rwall), whereina solid state light emitter area ratio (ρ_(SSL)) is defined as the ratiobetween the area of the top of the at least one solid state lightemitter and the sum of the area of reflective surface of the base andthe area of the reflective wall surface, and wherein a largest linearsize (d_(SSL)) of the top surface of the at least one solid state lightemitter is defined as the longest distance from a point on the topsurface of the at least one solid state light emitter to another pointon the top surface of the at least one solid state light emitter along astraight line, and a partially diffusive reflective layer, comprising aluminescent material for converting at least a part of the light of thefirst color into light of a second color, the light exit windowcomprising at least a part of the partially diffusive reflective layer,wherein a gap with a distance h is present between the top surface ofthe at least one solid state light emitter and the partially diffusivereflective layer for which 0.3·d_(SSL)≦h≦0.75·d_(SSL) for 0<ρ_(SSL)<0.1,0.15·d_(SSL)≦h≦0.3·d_(SSL) for 0.1≦ρ_(SSL)≦0.25, and0.1·d_(SSL)≦h≦0.2·d_(SSL) for ρ_(SSL)>0.25, and wherein the value of theeffective reflection coefficient (Reff) is larger than 70% and largerthan the solid state light emitter reflection coefficient (R_SSL) andwherein the wall reflection coefficient (Rwall) is smaller than 95%. 2.A light emitting module according to claim 1, comprising a plurality ofsolid state light emitters, wherein each one of the solid state lightemitters is configured for emitting light in a specific color and eachone of the solid state light emitters having a top surface, and whereinthe solid state light emitter reflection coefficient (R_SSL) is definedas the average value of the reflection coefficients of the plurality ofsolid state light emitters.
 3. A light emitting module according toclaim 1 wherein the value of the effective reflection coefficient (Reff)is larger than the solid state light emitter reflection coefficient(R_SSL) plus a factor c times the difference between 1 and the solidstate light emitter reflection coefficient (R_SSL), wherein 0.2≦c ≦1 for0<ρ_(SSL)<0.1, 0.3≦c≦1 for 0.1≦ρ_(SSL)≦0.25, and 0.4≦c≦1 forρ_(SSL)>0.25.
 4. A light emitting module according to claim 1, whereinat least a part of the reflective surface of the base is closer to thepartially diffusive reflective layer than the top surface of the atleast one solid state light emitter, and wherein the distance h betweenthe top surface and the partially diffusive reflective layer is 0.4*d_(SSL)+Δh/2≦h≦5* d_(SSL)+Δh/2for 0<ρ_(SSL)<0.1, 0.15* d_(SSL)+Δh/2≦h≦3*d_(SSL)+Δh/2for 0.1≦ρ_(SSL)≦0.25, and 0.1* d_(SSL)+Δh/2≦h≦2*d_(SSL)+Δh/2 for ρ_(SSL)>0.25, and wherein Δh is the absolute value ofthe difference between the distance (h) between the top surface of theat least one solid state light emitter and the partially diffusivereflective layer and the shortest distance between the reflective basesurface and the partially diffusive reflective layer.
 5. A lightemitting module according to claim 1, wherein the light reflective wallsurface is tilted with respect to a normal axis of the base wherein thetilt increases the reflection of light towards the light exit window. 6.A light emitting module according to claim 1, wherein the partiallydiffusive reflective layer forms the light exit window, the partiallydiffusive reflective layer having an edge, and the edge of the partiallydiffusive reflective layer being in contact with the base.
 7. A lightemitting module according to claim 1, comprising a substantiallytransparent material arranged between the at least one solid state lightemitter and the partially diffusive reflective layer, the transparentmaterial being optically coupled to the at least one solid state lightemitter.
 8. A light emitting module according to claim 7 wherein thesubstantially transparent material is further optically and thermallycoupled to the partially diffusive reflective layer.
 9. A light emittingmodule according to claim 7 wherein the substantially transparentmaterial is sintered translucent polycrystalline alumina with a grainsize that is larger than 44 μm or smaller than 1 μm.
 10. A lightemitting module according to claim 1, wherein the light exit windowfurther comprises a diffuser layer for obtaining a diffuse lightemission, for obtaining a spatially, color and color over-angle uniformlight emission, and for obtaining a color mixed light emission.
 11. Alight emitting module according to claim 1, wherein a diffuser layer forobtaining a diffuse light emission, for obtaining a spatially, color andcolor over-angle uniform light emission, and for obtaining a color mixedlight emission is provided at a distance from a side of the partiallydiffusive reflective layer facing away from the at least one solid statelight emitter.
 12. A lamp comprising a light emitting module accordingto claim
 1. 13. A luminaire comprising a light emitting module accordingto claim 1 or comprising a lamp.
 14. A display device comprising a lightemitting module according to claim
 1. 15. A light emitting moduleaccording to claim 1 wherein the light reflective wall surface is curvedfor increasing the reflection of light towards the light exit window.16. A light emitting module according to claim 1, wherein the light exitwindow further comprises a dichroic layer for correcting color overangle variations or light uniformity.
 17. A light emitting moduleaccording to claim 1, wherein the light exit window further comprises anoptical element for providing a desired light beam shape.